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.
You'll need to surround "fauxfactory.constants.VALID_NETMASKS" with backticks. That didn't show up in my earlier comment. (whoops)