Make sure the Puppet certificate name is a valid hostname or FQDN

The certname field is intended to submit a hostname other than
the actual hostname of the system in case it's desirable to
identify the node by a different name. Previously, we didn't
validate this. Do so.

Change-Id: I19ce15a30dad9487fff41fdc9aeb2c9f58c04e82
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=987793
Signed-off-by: Ryan Barry <rbarry@redhat.com>