Merge DB initialization scripts

[stefan-k]

There are currently two scripts which initialize the necessary DBs: init_db.sh and init_slurm_collector_sqlite.sh. Maybe think about merging the two and update the documentation accordingly.

[QuantumDancer]

Not sure if this makes sense, as we might use only one or the other script in some cases. For example, for integration tests such as the one for the priority plugin:

AUDITOR/scripts/test_priority_plugin.sh

Line 185 in 281d697

SKIP_DOCKER=true POSTGRES_DB=$DB_NAME ./scripts/init_db.sh

[stefan-k]

I was thinking about flags with which one can choose what the script should do. AFAIR there is code duplication in the scripts?