In staging, the db is already created by puppet (with necessary users, etc...).
The current create database cli swh db create does not work currently. The
subsequent steps of postgresql extensions installation is required though (and
it's currently stuck on the creation step failing).
This commit allows to use the init-admin to just install those extensions (and
eventually add/update existing/new ones).
In future diffs, we could also fallback to the postgresql image ability to
create the database during its initialization (and then drop the current `if db
exists` instruction in our entrypoint scripts). D4377 demonstrates such a
Related to T2736