Add upgrade_if_available on config_changed

- reactive charms like aodh, barbican and etc uses the
  charms.openstack defaults for common states and hooks.
  The config-changed hook on the OpenStackCharm is missing a way
  of upgrading after changing the openstack-origin config, so the
  charm doesn't upgrade the packages.

- Without this, the charm can eventually upgrade if it needs to
  refresh the packages or if the user manually running the
  openstack-upgrade, but this is not an expected behavior.

Closes-Bug: #2039604
Change-Id: I35dd06c0cf2810599e2988536e22177e06ca2803

charms_openstack/charm/classes.py

@@ -443,6 +443,7 @@ class OpenStackCharm(BaseOpenStackCharm,
     def config_changed(self):
         tls = reactive.endpoint_from_flag('certificates.available')
         self.configure_tls(certificates_interface=tls)
+        self.upgrade_if_available(interfaces_list=None)
 
     def configure_ssl(self, keystone_interface=None):
         """DEPRECATED Configure SSL certificates and keys.

unit_tests/charms_openstack/charm/test_classes.py

@@ -476,8 +476,10 @@ class TestMyOpenStackCharm(BaseOpenStackCharmTest):
 
     def test_config_changed(self):
         self.patch_target('configure_tls')
+        self.patch_target('upgrade_if_available')
         self.target.config_changed()
         self.configure_tls.assert_called_once_with(certificates_interface=None)
+        self.upgrade_if_available.assert_called_once_with(interfaces_list=None)
 
         self.configure_tls.reset_mock()
         ep_mock = mock.MagicMock()