Deal with py2-py3 migration during charm upgrade

On charm upgrade the charm may switch to py3 packages. If so, ensure
the old py2 packages are purged after any new packages have been
installed.  If packages where removed during the upgrade, then
restart all services to pickup any py2->py3 switches.

Change-Id: I8a17f679e85448680ab0b1f5ae362ef7a65b035c
Closes-Bug: 1803451

hooks/keystone_hooks.py

@@ -123,6 +123,7 @@ from keystone_utils import (
     key_write,
     pause_unit_helper,
     resume_unit_helper,
+    remove_old_packages,
 )
 
 from charmhelpers.contrib.hahelpers.cluster import (
@@ -707,6 +708,7 @@ def configure_https():
 def upgrade_charm():
     status_set('maintenance', 'Installing apt packages')
     apt_install(filter_installed_packages(determine_packages()))
+    packages_removed = remove_old_packages()
 
     if run_in_apache():
         disable_unused_apache_sites()
@@ -718,6 +720,11 @@ def upgrade_charm():
 
     update_nrpe_config()
 
+    if packages_removed:
+        log("Package purge detected, restarting services", "INFO")
+        for s in services():
+            service_restart(s)
+
     if is_elected_leader(CLUSTER_RES):
         log('Cluster leader - ensuring endpoint configuration is up to '
             'date', level=DEBUG)