Pull Request - Add Xenserver to list of supported OS families

I thought I posted previously, but either my post is under review, or I
didn't actually hit "post". Hopefully this isn't a dupe.

I've created a pull request to add Xenserver OS family support to Foreman,
based on mmoll's original work:
https://github.com/theforeman/foreman/pull/3331. I haven't contributed
before, so I apologize ahead of time if I'm not following procedure
correctly. I'm following along here: Foreman :: Contribute

I didn't add additional Xenserver provision/PXE templates. I can if that's
desirable. Xenserver requires a few extra files for its PXE config, so I
thought I'd add it as a separate template, rather than edit

Related Redmine issues: