lxd provisioner needs to respect SetContainerLinkLayerDevices errors

[John A Meinel]

With my patch here:
  https://github.com/juju/juju/pull/6733/files

If I try to do:
  juju add-machine lxd:0 --constraints spaces=INVALID

It still creates the machine and provisions a 0/lxd/2 machine. However that machine only comes up on LXDBR0, the expected result is to have a provisioning failure.

Note also bug #1650254 and bug #1650253. This is slightly different context. It may just be the same as one of those bugs, though.

[John A Meinel (jameinel)]

lxd-broker currently reports the error, but keeps going:
2016-12-19 15:05:57 WARNING juju.provisioner lxd-broker.go:62 failed to prepare container "0/lxd/1" network config: unable to find host bridge for spaces [guest-internal] for container "0/lxd/1"
2016-12-19 15:05:57 WARNING juju.provisioner broker.go:97 incomplete DNS config found, discovering host's DNS config

It just treats the failure to find a bridge as a warning, and then puts the container on "lxdbr0".

[Joseph Phillips (manadart)]

This behaviour has changed since reported.