Use network's mac prefix

Modify existing GenerateMAC so that it takes network as an argument.

Substitute _GenerateOneMAC with a decorator _GenerateMACPrefix that
chooses the prefix (network's or cluster's) and wraps
_GenerateMACSuffix that generates the three remaining bytes.

Check mac prefix validity when adding/modifing a network.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
Reviewed-by: Iustin Pop <iustin@google.com>