(FACT-179) Resolve fqdn fact when domain is empty

Previously, the fqdn fact would only resolve if both hostname and
domain were available on a host. Having a nil result when `hostname
-f` would return just the hostname is surprising to users, and we
should instead simply return the hostname when we cannot resolve a
domain.
Merge branch 'pr/687' into facter-2

* pr/687:
  (FACT-179) Resolve fqdn fact when domain is empty