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.



Like maas-agent-name above, I believe the default needs to be "" to cater for existing environments that don't have the network-bridge setting. You would then need to update networkBridge() to check if it doesn't exist/is empty, and returns eth0 instead.