Enable registration of packages with principal charm

For principal - subordinate plugin type relations where the
principal Python payload imports code from packages managed by a
subordinate, upgrades can be problematic.

This change will allow a subordinate charm that have opted into the
feature to inform its principal about all implemented release -
packages combinations ahead of time. With this information in place
the principal can do the upgrade in one operation without risk of
charm relation RPC type processing at a critical moment.

Depends-On: https://review.opendev.org/c/openstack/charms.openstack/+/781487
Depends-On: https://review.opendev.org/c/openstack/charm-layer-openstack/+/781624
Depends-On: https://review.opendev.org/c/openstack/charm-interface-keystone-domain-backend/+/781658
Closes-Bug: #1806111
Change-Id: Ib3239690286dc902eb247edec96b8dca7bf5bee4

src/reactive/keystone_ldap_handlers.py

@@ -25,7 +25,9 @@ import charmhelpers.core.hookenv as hookenv
 
 charm.use_defaults(
     'charm.installed',
-    'update-status')
+    'update-status',
+    'upgrade-charm',
+)
 
 # if config has been changed we need to re-evaluate flags
 # config.changed is set and cleared (atexit) in layer-basic

unit_tests/test_keystone_ldap_handlers.py

@@ -24,7 +24,9 @@ class TestRegisteredHooks(test_utils.TestRegisteredHooks):
     def test_hooks(self):
         defaults = [
             'charm.installed',
-            'update-status']
+            'update-status',
+            'upgrade-charm',
+        ]
         hook_set = {
             'when': {
                 'configure_domain_name': ('domain-backend.connected',