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).
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).
Related to T2736