Extend default glance limit

Since [1] is merged, quota in glance is enabled but limit is set to
1GB by default and this is too small for Magnum jobs using Fedora
images.
This change extends the limit to fix the issue caused by that low
limit.

[1] https://review.opendev.org/c/openstack/devstack/+/788056
Depends-on: https://review.opendev.org/802413

Change-Id: Id741fafc7e1da2d4e180f0745cd4d4b367472f06

.zuul.yaml

@@ -27,6 +27,9 @@
               flavor_id: ds2G
               master_flavor_id: ds2G
               copy_logs: true
+      devstack_localrc:
+        # NOTE: extend default glance limit from 1GB
+        GLANCE_LIMIT_IMAGE_SIZE_TOTAL: 5000
 
 - job:
     name: magnum-tempest-plugin-base