Allow simple read-only connections to db from swh nodes

Our current recommended pg_service setup to connect to the DB replica is:

[swh-replica]
dbname=softwareheritage
host=somerset.internal.softwareheritage.org
user=guest
port=5433

But that needs one swh user to actually setup manually such file (and also be aware of what's what). We should puppetize this and install it by default on our swh nodes so that user can simply execute:

swhuser@swhnode $ psql service=swh-replica

for example to connect to the db replica. Related #1234 (closed)

Note: As a first incremental step, i'll only reference the guest user (read-only access). Shout out if we need also write access.

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Migrated from T1234 (view on Phabricator)