add netmask_to_prefixlen to salt.utils.network and make it available through network.netmask_to_prefixlen
Workaround b/c salt.utils.network isn't a refreshable module.

I couldn't figure out where this function should go to
(salt/modules/network.py or salt/utils/network.py) so I leave
this to the actual salt devs to sort out.
make pylint 0.09 points happier