Looks like juju detects correct settings from the machine (look for 192.168.33 subnet):

2017-11-21 00:51:24 DEBUG juju.worker.machiner machiner.go:172 observed network config updated for "machine-16" to [{1  127.0.0.0/8 65536      0 lo  loopback false false loopback 127.0.0.1 [] []  []} {1  ::1/128 65536      0 lo  loopback false false loopback ::1 [] []  []} {2 14:02:ec:42:38:dc  1500      0 eno1  ethernet false false manual  [] []  []} {3 5c:b9:01:9a:3c:88 10.245.208.0/20 1500      0 eth4  ethernet false false static 10.245.209.20 [] []  []} {3 5c:b9:01:9a:3c:88  1500      0 eth4  ethernet false false manual  [] []  []} {4 14:02:ec:42:38:dd  1500      0 eno2  ethernet false false manual  [] []  []} {5 14:02:ec:42:38:de  1500      0 eno3  ethernet false false manual  [] []  []} {6 5c:b9:01:9a:3c:89  9000      0 eth1  ethernet false false manual  [] []  []} {7 14:02:ec:42:38:df  1500      0 eno4  ethernet false false manual  [] []  []} {8 00:11:0a:66:2c:b4  9000      0 eth6  ethernet false false manual  [] []  []} {9 00:11:0a:66:2c:b4  9000      0 eth7  ethernet false false manual  [] []  []} {10 00:11:0a:66:2c:b4  9000      0 bond0  bond false false manual  [] []  []} {11 00:11:0a:66:2c:b4 192.168.35.0/24 9000      0 bond0.2735  802.1q false false static 192.168.35.8 [] []  []} {11 00:11:0a:66:2c:b4  9000      0 bond0.2735  802.1q false false manual  [] []  []} {12 00:11:0a:66:2c:b4 192.168.36.0/24 9000      0 bond0.2736  802.1q false false static 192.168.36.8 [] []  []} {12 00:11:0a:66:2c:b4  9000      0 bond0.2736  802.1q false false manual  [] []  []} {13 5c:b9:01:9a:3c:89 192.168.33.0/24 9000      0 eth1.2733  802.1q false false static 192.168.33.127 [] []  []} {13 5c:b9:01:9a:3c:89  9000      0 eth1.2733  802.1q false false manual  [] []  []} {14 5c:b9:01:9a:3c:89  1500      0 eth1.2734  802.1q false false manual  [] []  []} {15 6a:43:a8:e2:cb:49  1500      0 lxdbr0  bridge false false manual  [] []  []} {15 6a:43:a8:e2:cb:49  1500      0 lxdbr0  bridge false false manual  [] []  []}]

2017-11-21 00:55:14 DEBUG juju.provisioner.lxd host_preparer.go:111 updating observed network config for "machine-16" to []params.NetworkConfig{...
params.NetworkConfig{DeviceIndex:22, MACAddress:"5c:b9:01:9a:3c:89", CIDR:"192.168.33.0/24", MTU:9000, ProviderId:"", ProviderSubnetId:"", ProviderSpaceId:"", ProviderAddressId:"", ProviderVLANId:"", VLANTag:0, InterfaceName:"br-eth1.2733", ParentInterfaceName:"", InterfaceType:"bridge", Disabled:false, NoAutoStart:false, ConfigType:"static", Address:"192.168.33.127", DNSServers:[]string(nil), DNSSearchDomains:[]string(nil), GatewayAddress:"", Routes:[]params.NetworkRoute(nil)}...

But however, creates a container with wrong MTU:

2017-11-21 00:56:32 DEBUG juju.cloudconfig.containerinit container_userdata.go:73 generating network config from container.NetworkConfig{NetworkType:"bridge", Device:"lxdbr0", MTU:0, Interfaces:[]network.Interfa
ceInfo{network.InterfaceInfo{DeviceIndex:0, MACAddress:"00:16:3e:4d:a2:ff", CIDR:"192.168.33.0/24", ProviderId:"227", ProviderSubnetId:"4", ProviderNetworkId:"", ProviderSpaceId:"", ProviderVLANId:"5037", Provid
erAddressId:"409", AvailabilityZones:[]string(nil), VLANTag:2733, InterfaceName:"eth0", ParentInterfaceName:"br-eth1.2733", InterfaceType:"ethernet", Disabled:false, NoAutoStart:false, ConfigType:"static", Addre
ss:local-cloud:192.168.33.141, DNSServers:[]network.Address{local-cloud:10.245.208.33, local-cloud:10.245.208.31, local-cloud:10.245.208.32}, MTU:1500, DNSSearchDomains:[]string{"maas"}, GatewayAddress:, Routes:
[]network.Route{}}, network.InterfaceInfo{DeviceIndex:1, MACAddress:"00:16:3e:40:fe:1e", CIDR:"10.245.208.0/20", ProviderId:"228", ProviderSubnetId:"2", ProviderNetworkId:"", ProviderSpaceId:"", ProviderVLANId:"
5035", ProviderAddressId:"411", AvailabilityZones:[]string(nil), VLANTag:0, InterfaceName:"eth1", ParentInterfaceName:"br-eth4", InterfaceType:"ethernet", Disabled:false, NoAutoStart:false, ConfigType:"static", 
Address:local-cloud:10.245.209.36, DNSServers:[]network.Address{}, MTU:1500, DNSSearchDomains:[]string(nil), GatewayAddress:local-cloud:10.245.208.1, Routes:[]network.Route{}}}}
2017-11-21 00:56:32 DEBUG juju.cloudconfig.containerinit container_userdata.go:258 prepared network config for rendering: &{InterfaceNames:[lo {eth00_16_3e_4d_a2_ff} {eth00_16_3e_40_fe_1e}] AutoStarted:[lo {eth0
0_16_3e_40_fe_1e} {eth00_16_3e_4d_a2_ff}] DNSServers:[10.245.208.31 10.245.208.32 10.245.208.33] DNSSearchDomains:[maas] NameToAddress:map[{eth00_16_3e_4d_a2_ff}:192.168.33.141/24 {eth00_16_3e_40_fe_1e}:10.245.2
09.36/20] NameToRoutes:map[{eth00_16_3e_4d_a2_ff}:[] {eth00_16_3e_40_fe_1e}:[]] NameToMTU:map[] Gateway4Address:10.245.208.1 Gateway6Address:}
2017-11-21 00:56:32 DEBUG juju.cloudconfig.containerinit container_userdata.go:163 generated network config:

auto lo {eth00_16_3e_40_fe_1e} {eth00_16_3e_4d_a2_ff}

iface lo inet loopback
  dns-nameservers 10.245.208.31 10.245.208.32 10.245.208.33
  dns-search maas

iface {eth00_16_3e_4d_a2_ff} inet static
  address 192.168.33.141/24

iface {eth00_16_3e_40_fe_1e} inet static
  address 10.245.209.36/20
  gateway 10.245.208.1

2017-11-21 00:56:32 DEBUG juju.service discovery.go:63 discovered init system "systemd" from series "xenial"
2017-11-21 00:56:32 INFO juju.cloudconfig userdatacfg_unix.go:410 Fetching agent: curl -sSfw 'tools from %{url_effective} downloaded: HTTP %{http_code}; time %{time_total}s; size %{size_download} bytes; speed %{
speed_download} bytes/s ' --connect-timeout 20 --noproxy "*" --insecure -o $bin/tools.tar.gz <[https://10.245.208.202:17070/model/533c394c-7413-4d6f-8357-e9e8f2e65005/tools/2.2.6-xenial-amd64]>
2017-11-21 00:56:32 INFO juju.container.lxd lxd.go:176 instance "juju-e65005-16-lxd-6" configured with map[eth0:map[type:nic nictype:bridged name:eth0 parent:br-eth1.2733 hwaddr:00:16:3e:4d:a2:ff mtu:1500] eth1:
map[nictype:bridged name:eth1 parent:br-eth4 hwaddr:00:16:3e:40:fe:1e mtu:1500 type:nic]] network devices
2017-11-21 00:56:32 INFO juju.container.lxd lxd.go:187 starting instance "juju-e65005-16-lxd-6" (image "juju/xenial/amd64")...
2017-11-21 00:56:43 INFO juju.provisioner provisioner_task.go:790 started machine 16/lxd/6 as instance juju-e65005-16-lxd-6 with hardware "availability-zone=default", network config [{DeviceIndex:0 MACAddress:00:16:3e:4d:a2:ff CIDR:192.168.33.0/24 MTU:1500 ProviderId:227 ProviderSubnetId:4 ProviderSpaceId: ProviderAddressId:409 ProviderVLANId:5037 VLANTag:2733 InterfaceName:eth0 ParentInterfaceName:br-eth1.2733 InterfaceType:ethernet Disabled:false NoAutoStart:false ConfigType:static Address:192.168.33.141 DNSServers:[10.245.208.33 10.245.208.31 10.245.208.32] DNSSearchDomains:[maas] GatewayAddress: Routes:[]} {DeviceIndex:1 MACAddress:00:16:3e:40:fe:1e CIDR:10.245.208.0/20 MTU:1500 ProviderId:228 ProviderSubnetId:2 ProviderSpaceId: ProviderAddressId:411 ProviderVLANId:5035 VLANTag:0 InterfaceName:eth1 ParentInterfaceName:br-eth4 InterfaceType:ethernet Disabled:false NoAutoStart:false ConfigType:static Address:10.245.209.36 DNSServers:[] DNSSearchDomains:[] GatewayAddress:10.245.208.1 Routes:[]}], volumes [], volume attachments map[], subnets to zones map[]