EthernetDeviceForBridge fails if bridge has external addresses

[Federico Bosi]

I have a host with multiple ips on its bridge: (for example)

my:prefix:subnet::dhcp:ip/128 global dynamic
my:prefix:subnet2::static:ip/64 global

I have configured juju to only manage the statically assigned addresses on subnet2/64
`juju list-subnets` correctly lists my:prefix:subnet2::/64

When juju calls linklayerdevices.go:EthernetDeviceForBridge it tries to get the subnet
for the first address on the bridge (the dynamic one) and since the mask is /128 it fails
with "retrieving subnet my:prefix:subnet::dhcp:ip/128 used by address my:prefix:subnet::dhcp:ip/128
of host machine device br-ens2".

I made it loop over the devices and return the first valid one, and it works.
I'll put a pull request on github shortly and would like your feedback as I'm new to open source :)

[Joseph Phillips (manadart)]

https://github.com/juju/juju/pull/17015