Document registering to an entitlement server with a UUID as not implemented.

It was my understanding that we would be able to pass in an existing UUID
when registering to connect to an existing registration (for the case where
IPA is re-installed). This is supported in the REST API but not python-rhsm.

https://fedorahosted.org/freeipa/ticket/1216

ipalib/plugins/entitle.py

@@ -488,7 +488,7 @@ class entitle_register(LDAPCreate):
     takes_options = LDAPCreate.takes_options + (
         Str('ipaentitlementid?',
             label='UUID',
-            doc=_('Enrollment UUID'),
+            doc=_('Enrollment UUID (not implemented)'),
             flags=['no_create', 'no_update'],
         ),
         Password('password',
@@ -516,6 +516,9 @@ class entitle_register(LDAPCreate):
         os.environ['LANG'] = 'en_US'
         locale.setlocale(locale.LC_ALL, '')
 
+        if 'ipaentitlementid' in options:
+            raise errors.ValidationError(name='ipaentitlementid', error='Registering to specific UUID is not supported yet.')
+
         try:
             registrations = api.Command['entitle_find']()
             raise errors.AlreadyRegisteredError()