staging monitoring: Fix vhost computation to use public vhost
The [webapp|deposit].internal.staging.s.n servers are no longer running hitch/varnish and should not be used as monitoring entrypoints. This commit removes them from the certificate aliases and declares a new alias for the reverse-proxy.
Related to T2747
Test Plan
octocatalog-diff [pergamon|rp0.staging|deposit.staging|webapp.staging] Not paste because quite big for the webapp/deposit (renaming of the vhosts)
Migrated from D4456 (view on Phabricator)