Ensure os_release is reset during upgrades

Reset the os_release cache during the OpenStack upgrade process,
ensuring that any post dist-upgrade operations are made in the
context of the new OpenStack release, not the old one.

Change-Id: I3d3584dd8e97f85e16c38e1143f627b03fa63bd0
Closes-Bug: 1715624

hooks/keystone_utils.py

@@ -74,6 +74,7 @@ from charmhelpers.contrib.openstack.utils import (
     make_assess_status_func,
     os_application_version_set,
     CompareOpenStackReleases,
+    reset_os_release,
 )
 
 from charmhelpers.contrib.python.packages import (
@@ -585,6 +586,7 @@ def do_openstack_upgrade(configs):
         '--option', 'Dpkg::Options::=--force-confdef',
     ]
     apt_upgrade(options=dpkg_opts, fatal=True, dist=True)
+    reset_os_release()
     apt_install(packages=determine_packages(), options=dpkg_opts, fatal=True)
 
     # set CONFIGS to load templates from new release and regenerate config

unit_tests/test_keystone_utils.py

@@ -68,6 +68,7 @@ TO_PATCH = [
     'pwgen',
     'os_application_version_set',
     'is_leader',
+    'reset_os_release',
 ]
 
 openstack_origin_git = \
@@ -194,6 +195,7 @@ class TestKeystoneUtils(CharmTestCase):
         self.assertTrue(configs.write_all.called)
         self.assertTrue(migrate_database.called)
         disable_unused_apache_sites.assert_called_with()
+        self.reset_os_release.assert_called()
 
     def test_migrate_database(self):
         self.os_release.return_value = 'havana'