BZ #1154669: Allow host to be set to existing hostgroup

https://bugzilla.redhat.com/show_bug.cgi?id=1154669

If the host is already associated to a deployment, this allows the host
to be edited directly in foreman, and to keep the hostgroup the same. It
will not allow any other deployment hostgroups.
Merge pull request #354 from bcrochet/bz/1154669

BZ #1154669: Allow host to be set to existing hostgroup