Make the memcache flush optional

The memcache flush implemented in https://review.openstack.org/608066
is actually a workaround. The implementation should really be
implemented more surgically in keystone itself. This has begun with
https://review.openstack.org/612686 but it is not complete. However,
something that's been made clear by the team is that this was only
required for the Newton->Ocata upgrade and has not been required since.

This workaround may be required again in the future, so instead of
removing it, we make the tool opt-in via a toggle which can then easily
be set when doing the appropriate major upgrade.

Related-Bug: 1793389
Change-Id: Ied0ce1e9877697bb627f784a0590f7c7e924479b

defaults/main.yml

@@ -511,3 +511,9 @@ keystone_extra_headers: []
 
 # List of trusted IPs which can pass X-Forwarded-For
 keystone_set_real_ip_from: []
+
+# Toggle whether memcache should be flushed when doing
+# database migrations. This is sometimes useful when
+# doing upgrades, but should not usually be required.
+# ref: https://bugs.launchpad.net/openstack-ansible/+bug/1793389
+keystone_flush_memcache: no

handlers/main.yml

@@ -174,6 +174,8 @@
   delegate_to: "{{ item }}"
   with_items: "{{ groups.memcached_all }}"
   listen: flush cache
+  when:
+    - keystone_flush_memcache | bool
 
 - meta: noop
   listen: Manage LB