In gerrit.config we can now define a list of mandatory plugins:
[plugins]
mandatory = replication
mandatory = hooks
If a mandatory plugin does not load, either because it is not present in
the plugins directory or because it fails to load, Gerrit start will
fail.
Change-Id: I26c578051b0f91844bab0a9b46e0fac34d88a15e
350fc68114