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

RHEL always use the FQDN as hostname thats the default
behavior, adding the config rhel_use_hostname will force
RHEL to use the hostname if sent

LP: canonical#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 util.get_cfg_option_bool(self._cfg, "rhel_use_hostname", False) \
+           and hostname:
+            return hostname
         if fqdn:
             return fqdn
         return hostname