Update overlay

* Remove prometheus-ceph-exporter
* Update mongodb charm to fix nrpe check
* Test of charmhub overlay format

config/overlays/openstack_versioned_overlay_focal.yaml

@@ -1,3 +1,7 @@
+variables:
+  charm-series:    &charm-series    focal
+  charm-arch:      &charm-arch      amd64
+
 applications:
   aodh:
     charm: cs:aodh-48
@@ -44,7 +48,7 @@ applications:
   graylog:
     charm: cs:~llama-charmers-next/graylog-19
   graylog-mongodb:
-    charm: cs:mongodb-59
+    charm: cs:mongodb-67
   hacluster-aodh:
     charm: cs:hacluster-76
   hacluster-cinder:
@@ -142,7 +146,11 @@ applications:
   nova-cloud-controller:
     charm: cs:~openstack-charmers-next/nova-cloud-controller-549 # bug LP#1928992 Placement endpoints not being updated, or perhaps regressing to n-c-c endpoints, even after deploying placement service for train
   nova-compute:
-    charm: cs:nova-compute-327
+    charm: nova-compute
+    channel: "ussuri/edge"
+    revision: 550
+    architecture: *charm-arch
+    series: *charm-series
   nrpe-container:
     charm: cs:nrpe-73
   nrpe-host:
@@ -158,8 +166,6 @@ applications:
     charm: cs:~llama-charmers-next/openstack-service-checks-12
   prometheus:
     charm: cs:prometheus2-22
-  prometheus-ceph-exporter:
-    charm: cs:prometheus-ceph-exporter-13
   prometheus-openstack-exporter:
     charm: cs:prometheus-openstack-exporter-17
   rabbitmq-server: