Adding license checks for Tower inventory source

* For Tower the license must match between the source and destination
* For AWX the check is disabled
* Hosts imported from another Tower don't count against your license
  in the local Tower
* Fix up some issues with enablement
* Prevent slashes from being used in the instance filter
* Add &all=1 filter to make sure we pick up all hosts

awx/main/tests/functional/test_credential.py

@@ -29,6 +29,7 @@ def test_default_cred_types():
         'satellite6',
         'scm',
         'ssh',
+        'tower',
         'vault',
         'vmware',
     ]