Read config from a central site config file rather from the environment

Use the module hzb_rdm.config from python-hzb-rdm to read a configuration file rather than taking it from the environment. As of now, the configuration needed in the backend include the parameters needed to connect to the database and the backend API base. The latter is only needed in the build_database script. Further configuration settings may be added as needed. Note that the configuration file may need to be shared with other applications.

Note that this MR also needs to modify the build_database script, it is build on top of !7 (merged) and thus depends on that other MR.