Merge pull request #12433 from hmlanigan/charmhub-caas-common-facade

#12433

Add CharmHub to the set commonModelFacadeNames. To allow k8s controllers to deploy CharmHub charms.

## QA steps

Steps assume that microk8s and docker is setup already.

```console
$ make microk8s-operator-update
$ juju bootstrap microk8s wednesday-k8s
$ export JUJU_DEV_FEATURE_FLAGS="charm-hub"
$ juju add-model six --config charm-hub-url="https://api.staging.snapcraft.io"

# the find command should not fail
$ juju find --series kubernetes

# deploy a charmhub k8s charm
$ juju deploy postgresql-k8s
$ juju status
Model Controller Cloud/Region Version SLA Timestamp
six wednesday-k8s microk8s/localhost 2.9-rc3 unsupported 14:33:24Z

App Version Status Scale Charm Store Rev OS Address Message
postgresql pgcharm:edge active 1 postgresql-k8s charmhub 5 kubernetes

Unit Workload Agent Address Ports Message
postgresql/0* active idle 10.1.246.77 5432/TCP Pod configured

$ juju show-status-log postgresql/0
Time Type Status Message
...
09 Dec 2020 14:32:45Z juju-unit idle
09 Dec 2020 14:32:45Z workload active Pod configured
```

apiserver/restrict_caasmodel.go

@@ -22,6 +22,7 @@ var commonModelFacadeNames = set.NewStrings(
 	"Block",
 	"CharmRevisionUpdater",
 	"Charms",
+	"CharmHub",
 	"Cleaner",
 	"Client",
 	"Cloud",