Simplify mac generation funtions

Introduce new config method _UnlockedGetNetworkMACPrefix and refactor
original _GenerateOneMAC so that it takes prefix as an optional
argument and returns a function that actually generates a MAC. If
prefix is not given or None it uses the cluster level default.

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