Add cluster netmask parameter

Add the master_netmask cluster parameter, that represents the netmask of
the master IP, encoded as a CIDR suffix.

This parameter can be set via the --master-netmask of gnt-cluster init
and gnt-cluster modify. The default behaviour is to be consistent with
the old default (/32 for IPv4 and /128 for IPv6).

Signed-off-by: Andrea Spadaccini <spadaccio@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>