GH-2087: Update host.rb to detect RHEL (in addition to CentOS and Fedora)

This patch adds RHEL support to `plugins/hosts/fedora/host.rb`.

Note that this patch already includes the fix to issue https://github.com/mitchellh/vagrant/issues/2088 (Update host.rb to use proper capture group for OS version regex of RHEL family).

If needed I can also rebase this pull request on vagrant's master branch.  However the regex used in master is broken anyways as of now because of the incorrect regex capture group, so it doesn't seem to make a lot of sense not to fix issue https://github.com/mitchellh/vagrant/issues/2088 first.