Use mysql memory reduction flags for ceph job

This makes the ceph-multistore job use the MYSQL_REDUCE_MEMORY
flag in devstack to try to address the frequent OOMs we see in that
job.

Conflicts:
  .zuul.yaml

Change-Id: Ibc203bd10dcb530027c2c9f58eb840ccc088280d
Closes-Bug: #1961068
(cherry picked from commit 84d1f25)

Author: kk7ds

.zuul.yaml

@@ -585,6 +585,7 @@
         GLANCE_USE_IMPORT_WORKFLOW: True
         DEVSTACK_PARALLEL: True
         CEPH_MIN_CLIENT_VERSION: "mimic"
+        MYSQL_REDUCE_MEMORY: True
       # NOTE(danms): This job is pretty heavy as it is, so we disable some
       # services that are not relevant to the nova-glance-ceph scenario
       # that this job is intended to validate.