bionic LXD containers with more than one space configures multiple default gateways using netplan (xenial/ifupdown does not)

When deploying bionic LXD containers with more than 1 space (e.g. default space + application binding space) multiple default gateways are set. This is not the case with xenial LXD containers.

Looking at the LXD configuration, we see that the netplan stanza outputs gateway4 for each interface but the ifupdown section only outputs gateway for one interface. It is expected this gateway will try to match the hosts default gateway interface if possible (Since Pull #8140).

Tested on Juju 2.4.3-cosmic-amd64

lxd config show excerpt:

==Netplan ==

        version: 2
        ethernets:
          eth0:
            match:
              macaddress: 00:16:3e:e3:fb:3a
            addresses:
            - 100.64.0.59/24
            gateway4: 100.64.0.254
            nameservers:
              search: [maas]
              addresses: [100.64.0.253]
            mtu: 9000
          eth1:
            match:
              macaddress: 00:16:3e:fd:8d:5c
            addresses:
            - 100.64.1.44/24
            gateway4: 100.64.1.254
            mtu: 9000

== ifupdown ==
      auto lo {eth00_16_3e_e3_fb_3a} {eth00_16_3e_fd_8d_5c}

      iface lo inet loopback
        dns-nameservers 100.64.0.253
        dns-search maas

      iface {eth00_16_3e_e3_fb_3a} inet static
        address 100.64.0.59/24
        gateway 100.64.0.254
        mtu 9000

      iface {eth00_16_3e_fd_8d_5c} inet static
        address 100.64.1.44/24
        mtu 9000