container/lxd: fallback to "default" LXD profile
container/lxd: fallback to "default" LXD profile

This was work originally started by Tycho in:

  https://github.com/juju/juju/pull/5136

but updated to consider the case where we have a default bridge from the
provider and we should use that before falling back to LXD's "default"
profile.

Original - PR: https://github.com/juju/juju/pull/5136

73a85aaf creates network profiles based on network configuration
provided to the container type when network spaces are present (e.g. in
maas, etc.). However, in some cases (notably, GCE), there is no network
configuration provided to the container type code, and thus the profile
gets created with no network devices.

Instead, let's fall back to the default network configuration in LXD
when nothing is explicitly provided by juju.

ericsnowcurrently/juju,makyo/juju,frankban/juju,dimitern/juju,marcmolla/juju,mikemccracken/juju,mjs/juju,axw/juju,mikemccracken/juju,howbazaar/juju,gabriel-samfira/juju,bac/juju,mikemccracken/juju,mjs/juju,reedobrien/juju,kat-co/juju,alesstimec/juju,reedobrien/juju,ericsnowcurrently/juju,waigani/juju,mikemccracken/juju,bac/juju,perrito666/juju,howbazaar/juju,ericsnowcurrently/juju,AdamIsrael/juju,mjs/juju,perrito666/juju,axw/juju,dimitern/juju,mjs/juju,bogdanteleaga/juju,kat-co/juju,macgreagoir/juju,dooferlad/juju,anastasiamac/juju,frankban/juju,frankban/juju,waigani/juju,alesstimec/juju,anastasiamac/juju,dooferlad/juju,AdamIsrael/juju,alesstimec/juju,dooferlad/juju,bz2/juju,bogdanteleaga/juju,bogdanteleaga/juju,dooferlad/juju,anastasiamac/juju,reedobrien/juju,dimitern/juju,macgreagoir/juju,frankban/juju,axw/juju,dimitern/juju,waigani/juju,kat-co/juju,mjs/juju,perrito666/juju,mjs/juju,howbazaar/juju,alesstimec/juju,bac/juju,gabriel-samfira/juju,bz2/juju,axw/juju,anastasiamac/juju,marcmolla/juju,mikemccracken/juju,mjs/juju,AdamIsrael/juju,kat-co/juju,bz2/juju,macgreagoir/juju,dimitern/juju,AdamIsrael/juju,marcmolla/juju,marcmolla/juju,gabriel-samfira/juju,reedobrien/juju,bac/juju,makyo/juju,makyo/juju,reedobrien/juju,frankban/juju,bz2/juju,gabriel-samfira/juju,kat-co/juju,bogdanteleaga/juju,ericsnowcurrently/juju,macgreagoir/juju,anastasiamac/juju,dooferlad/juju,makyo/juju,gabriel-samfira/juju,bz2/juju,alesstimec/juju,axw/juju,marcmolla/juju,perrito666/juju,howbazaar/juju,perrito666/juju,bac/juju,howbazaar/juju,waigani/juju,ericsnowcurrently/juju,macgreagoir/juju