Add valid netmask random generator

I was motivated to create this because this regex used to validate netmasks https://github.com/theforeman/foreman/blob/develop/lib/net/validations.rb#L8.

Also got some info at http://www.iplocation.net/tools/netmask.php.
How about making the VALID_NETMASKS all unicode from the start? In that case, there's no need to call _make_unicode.