Add 'network-bridge' option to MAAS configuration

I exposed the variable 'network-bridge'' to allow users to specify to which network interface
the bridge should be bound to.

MAAS provider uses 'eth0' as the default network interface used to build the bridge br0, on some scenarios like LP: #1271144  (p1p2, eth2, ..) this assertion is not true.



Comment needs fixing to reflect the new attr name