Read tagstore connection settings from env

[behas]

Currently, one must pass the db connection string when inserting data into the tagstore, for example:

tagpack-tool ingest packs -u postgresql://$USER:$PASSWORD@$DBHOST:$DBPORT/tagstore

We should keep that option but it would be more convenient to also support reading required parameters from os.environ so that this works:

tagpack-tool ingest packs

If params in os.environ are not set and no connection string is passed, the tool should issue a warning.