Add an option for rhel to force the use of hostname config

RHEL always use the FQDN as hostname thats the default
behavior, setting prefer_fqdn_over_hostname option to
to false will prefer the hostname over fqdn

LP: #859

cloudinit/distros/rhel.py

@@ -94,6 +94,9 @@ def _write_hostname(self, hostname, out_fn):
     def _select_hostname(self, hostname, fqdn):
         # Should be fqdn if we can use it
         # See: https://www.centos.org/docs/5/html/Deployment_Guide-en-US/ch-sysconfig.html#s2-sysconfig-network # noqa
+        if not util.get_cfg_option_bool(self._cfg, "prefer_fqdn_over_hostname", True) \
+           and hostname:
+            return hostname
         if fqdn:
             return fqdn
         return hostname