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.

Change-Id: Ibc203bd10dcb530027c2c9f58eb840ccc088280d
Closes-Bug: #1961068

Author: kk7ds

.zuul.yaml

@@ -608,6 +608,7 @@
         GLANCE_USE_IMPORT_WORKFLOW: True
         DEVSTACK_PARALLEL: True
         GLANCE_LIMIT_IMAGE_SIZE_TOTAL: 2048
+        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.