[2.4.0] juju-no-proxy variable is not applied due to legacy no-proxy value being auto-set by juju

It appears to be that "auto noproxy" behavior triggers an incorrect behavior with new juju-no-proxy value albeit the legacy one is set to an empty string.

On 2.4.0 all juju-{http,https,no}-proxy configs landed into apt-{http,https,no}-proxy judging by the apiserver log.

juju model-config -m controller logging-config='<root>=ERROR;unit=TRACE;juju.worker.proxyupdater=TRACE'

juju model-config -m controller juju-http-proxy=http://10.10.101.2:8000 juju-https-proxy=http://10.10.101.2:8000 juju-no-proxy='localhost,127.0.0.1,10.10.10.0/24,10.10.101.0/24' http-proxy='' https-proxy="" no-proxy=""

machine-0.log on the controller machine:

2018-07-17 20:27:14 DEBUG juju.worker.proxyupdater proxyupdater.go:185 new legacy proxy settings proxy.Settings{Http:"", Https:"", Ftp:"", NoProxy:"10.10.101.19", AutoNoProxy:""}
2018-07-17 20:27:14 TRACE juju.worker.proxyupdater proxyupdater.go:213 setting snap proxy values: proxy.Settings{Http:"", Https:"", Ftp:"", NoProxy:"", AutoNoProxy:""}, "", ""

# on a controller machine
ip -4 -o -br a s
lo UNKNOWN 127.0.0.1/8
enp1s0 UP 10.10.101.19/24

juju status -m controller
Model Controller Cloud/Region Version SLA Timestamp
controller vmaas vmaas 2.4.1 unsupported 23:44:37+03:00

Machine State DNS Inst id Series AZ Message
0 started 10.10.101.19 xwqr38 xenial default Deployed

snap info juju | grep -P ' candidate'
  candidate: 2.4.1+2.4-7213ffe (4691) 63MB classic

update for 2.4.0 (non-candidate):

cat bootstrap-config.yaml
juju-http-proxy: http://10.10.101.2:8000
juju-https-proxy: http://10.10.101.2:8000
juju-no-proxy: 'localhost,127.0.0.1,10.10.10.0/24,10.10.101.0/24'
logging-config: '<root>=ERROR;unit=TRACE;juju.worker.proxyupdater=TRACE'

juju bootstrap --config bootstrap-config.yaml --bootstrap-constraints mem=2048 vmaas

root@maas-vhost1:/var/log/juju# grep -i proxyupdate machine-0.log
2018-07-17 21:12:29 DEBUG juju.apiserver request_notifier.go:178 <- [1] machine-0 {"request-id":30,"type":"ProxyUpdater","version":2,"request":"ProxyConfig","params":"'params redacted'"}
2018-07-17 21:12:30 DEBUG juju.apiserver request_notifier.go:195 -> [1] machine-0 250.290772ms {"request-id":30,"response":"'body redacted'"} ProxyUpdater[""].ProxyConfig
2018-07-17 21:12:30 DEBUG juju.worker.logger logger.go:70 reconfiguring logging from "<root>=DEBUG" to "<root>=ERROR;unit=TRACE;juju.worker.proxyupdater=TRACE"
2018-07-17 21:12:34 DEBUG juju.worker.proxyupdater proxyupdater.go:165 new legacy proxy settings proxy.Settings{Http:"", Https:"", Ftp:"", NoProxy:"10.10.101.42,127.0.0.1,::1,localhost", AutoNoProxy:""}
2018-07-17 21:12:34 DEBUG juju.worker.proxyupdater proxyupdater.go:186 new apt proxy settings proxy.Settings{Http:"http://10.10.101.2:8000", Https:"http://10.10.101.2:8000", Ftp:"", NoProxy:"10.10.10.0/24,10.10.101.0/24,127.0.0.1,localhost", AutoNoProxy:""}
2018-07-17 21:12:34 DEBUG juju.worker.proxyupdater proxyupdater.go:165 new legacy proxy settings proxy.Settings{Http:"", Https:"", Ftp:"", NoProxy:"10.10.101.42,127.0.0.1,::1,localhost", AutoNoProxy:""}
2018-07-17 21:12:34 DEBUG juju.worker.proxyupdater proxyupdater.go:186 new apt proxy settings proxy.Settings{Http:"http://10.10.101.2:8000", Https:"http://10.10.101.2:8000", Ftp:"", NoProxy:"10.10.10.0/24,10.10.101.0/24,127.0.0.1,localhost", AutoNoProxy:""}

It appears to be that juju-http-proxy, juju-https-proxy and juju-no-proxy land into apt-http-proxy, apt-https-proxy and apt-no-proxy for some reason while they are not even set on the controller model:

juju model-config | grep proxy
apt-ftp-proxy default ""
apt-http-proxy default ""
apt-https-proxy default ""
apt-no-proxy default ""
ftp-proxy default ""
http-proxy default ""
https-proxy default ""
juju-ftp-proxy default ""
juju-http-proxy model http://10.10.101.2:8000
juju-https-proxy model http://10.10.101.2:8000
juju-no-proxy model localhost,127.0.0.1,10.10.10.0/24,10.10.101.0/24
logging-config model <root>=ERROR;unit=TRACE;juju.worker.proxyupdater=TRACE
no-proxy default 127.0.0.1,localhost,::1
proxy-ssh default false