Adds the 'maas-bridge-interface' option to MAAS configuration
	- By default 'eth0' is used, but this fails on some setups like LP: #1271144 when
	using (p1p2, eth2, ..) interfaces.
- Minor fixes according to @wallyworld observations
- Fixed according to @axw comment.
	- For backward-compatibility, network-bridge is an empty string by default "eth0" will be returned.
Merge pull request #227 from niedbalski/fix_1271144

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.