Some cleanup / simplification of tests was done in this PR:

• Remove duplicate things out of Dockerfiles and reorder stages for better use of cache
  • remove explicit installation of actinia-api and other requirements as it is done via actinia-core installation
• remove usage of ACTINIA_CUSTOM_TEST_CFG - test config can be used as normal config
• start removing overwrites of config in tests
• remove unused test file
• remove redis config - this is used e.g. here but the call of the main method is not used in our testing setup currently. But I don't see any necessary config option written there so I see no problem in testing that way in the future without that file
• leave a TODO in test_resource_base - handle it in a follow up PR when to decide how to handle ACTINIA_SERVER_TEST