improve build process, part of #481

fix oneJar code coverage issue (coverage info corrupts JAR), test built SlamEngine JAR, allow environment variable overrides of backend-config, modify travis to use local mongodb override, auto-import zips data set when in travis builds. note: publish script has not been updated and jenkins is no longer expected to work.
I'm not sure I understand. This function decides the name of the environment variable which must hold the configuration for the specified backend.

The JSON-encoded configuration is stored in the value of the environment variable, and will have different structure and encoding depending on the type of backend. In the MongoDB case, it will have structure, `{"mongodb": {"database": ..., "connectionUri": ...}}`.