Wladimir J. van der Laan
Make it possible to override bridge if for LXC
For example Ubuntu installs a bridge called `virbr0` by default for LXC.
Using this system-provided bridge avoids having to configure one
specifically before gitian building.
This commits makes it possible to override the bridge used by gitian
using the environment variable LXC_BRIDGE. The default is still to use
`br0` as before.