Set Ceph min client version to Mimic

Changes to the RBD Driver in Yoga leverage the RBD trash
functionality to enable Cinder users to delete volumes
that are still used in a dependency chain of Cinder volume
clones and in the future snapshots.

This relies on something purging the RBD trash.
In addition to this patch the user needs to enable the
rbd trash purge scheduling functionality. In this way the
Ceph backend will purge RBD images that are no longer used. 

Change-Id: Id8e581893ee4b373b268acc7c59b670985cedc2f

devstack/lib/ceph

@@ -484,6 +484,16 @@ function configure_ceph {
         sudo systemctl enable ceph-osd@${OSD_ID}
     done
 
+    if is_ceph_enabled_for_service cinder; then
+        # enables new features like Clone v2 API, which allows
+        # for proper handling of the deletion of snapshots
+        # with child clone images
+        if vercmp "$ceph_version" ">=" "13.0"; then
+            sudo ceph -c ${CEPH_CONF_FILE} \
+                osd set-require-min-compat-client mimic
+        fi
+    fi
+
     if is_ceph_enabled_for_service manila; then
         # create a MDS
         sudo mkdir -p ${CEPH_DATA_DIR}/mds/ceph-${MDS_ID}