Resolve "ro_user: grant execute only on defined functions"

Closes #33 (closed)

Test e.g. by

• create a thing with Frontend or tsm-dev-tools
• access Datastreams and Locations on STA Endpoint.
  • empty array should be returned
• connect to database with ro_credentials as found in tomcat context file and execute the following queries
  • select public.postgis_version();
  • select public.ST_ASGeoJSON(public.ST_SetSRID(public.ST_MakePoint(0, 0),4326));
  • select public.st_convexhull(public.ST_collect(public.ST_Makepoint(51.352478, 12.428764, 12.4)));

This should work without causing an error for either HTTP and the postgres query