Clean up PostgreSQL DB creation logic

[ekohl]

This started with the goal of getting rid of /root/.pgpass and instead use environment variables, but it escalated. See individual commits for details.

[evgeni]

ACK, but not going to merge, @ekohl might or might not want to squash my fixups into the right real commits ;)

[ekohl]

I did want to squash those up. I also extended the Drop unused zabbix_server_{images,data}_sql commit to do the same thing for MySQL. Made it easier to reason about in the tests :)