Compatibility with Slick extensions

- disabled tests, which use produce {fn …} calls which didn’t work for me in DB2
- disabled index reverse engineering in case of exception (Oracle requires extra permissions for indices, which may not always be available)
- allowed overriding dbPath from databases.properties for supporting remote test dbs (e.g. run tests on mac, connecting to dbs in windows vm)

@szeiger I'll merge this and #547 and possibly #579 and #580 and add some more docs and release RC1 by tonight, if you have no objections.