Check minimum size of networks on creation

When creating a network, so far no size constraints were checked.
We now limit the size of a network to a /30 or bigger, although
tecnically, the ipaddr library supports even /32 networks.

Signed-off-by: Helga Velroyen <helgav@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>