Set default constraint root-disk=20GB

Juju has an unwritten default root-disk size of 30GB which is used in
preference to the OpenStack flavors disk size when booting from volume
with root-disk-source=volume.

Set a default root-disk=20GB constraint to reduce the total storage
requirements of a default deployment, in line with what they were before
switching to booting from volumes by default in prodstack6.

common/generate_bundle_base

@@ -51,7 +51,7 @@ fi
 if [ -n "${MASTER_OPTS[MODEL_CONSTRAINTS]}" ]; then
     juju set-model-constraints ${MASTER_OPTS[MODEL_CONSTRAINTS]}
 else
-    juju set-model-constraints root-disk-source=volume
+    juju set-model-constraints root-disk-source=volume root-disk=20G
 fi
 
 if has_opt --list; then