Support LDAP authentication

[stefanigel]

As a user I'd like to use my existing LDAP/AD credentials to log into Superset.

This depends on the reference architecture developed in stackabletech/issues#170

[sbernauer]

Spike finished.
#145 for the Implementation
https://github.com/stackabletech/integration-tests/pull/175 for integration test
stackabletech/docker-images#64 for the docker image