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.
Can an issue be created requesting configurable netmasks? A /32 or /0 netmask is valid, and we should be able to generate them, even if those values seem nonsensical.