This is not removing the implicit configuration yet. This is just
adaptation to allow a similar behavior between other swh services.
In production, the actual environment variable is actually set (and
use the same value as before). So with this, at the next deployment,
we should see no difference.
The next step (later) would be to remove the implicit configuration
part (there are quite some ;).
Related T1533
Related D1161 (deposit, might be the nearest to the webapp)
Related D1167 D1178 D1170 rDSCHce90908cd0c32ce978ae813aa73235850cdeb4d2
Also Add missing dependency in requirements.txt (or docker build fails
without it). Docker adaptation will follow in another diff.