Reaplaced os.Symlink and os.Readlink

symlinks are not supported by standard go os package on windows. Replaced all instances of os.Symlink and os.Readlink with the implementation in github.com/juju/utils/symlink. On linux these functions simply call os.Symlink and os.Readlink. On windows they use our implementation
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.
Adds the 'network-bridge' option to MAAS configuration
   - By default 'eth0' is used, but this fails on some setups like LP: #1271144 when
    using (p1p2, eth2, ..) interfaces.