Gracefully handle existing default login roles If multiple databases are created in the same PostgreSQL cluster, existing roles are now properly re-used. This also fixes running database tests on a cluster that already has these roles, which was broken since rev. 5e38667f740c, because it silently fixed a test for existance of roles that failed if proper creation of the role 'sysadmin' failed during setup of the test database.

teichmann@intevation.de = sascha.teichmann@intevation.de
bernhard.reiter@intevation.de = bernhard@intevation.de