Improve gitlab pipelines

The https://git.gfz-potsdam.de/globaldynamicexposure/server-components/containers/docker-obm-database repo has been improved, so we can use it to run CI/CD pipelines with a PostGIS or SpatiaLite database installed, rather than building these each pipeline. In the MR following this issue we will implement them in exposure-lib.

In the MR, the following features are also improved:

• Error in pytest log message when using --db=spatialite or --db=postgis. The right tests would be run, but the message is the opposite (when using --db=spatialite, the message reads something like 'You need --db=spatialite to run)
• Mercantile is removed from the tests requirements in the setup.py, as it is a main requirement anyway
• The JSON indexing test is removed, as it is not very clear if this is the best way to test if the index is used.

\fyi @all