Keycloak Multi Tenants

This PR needs to be reviewed by Stian before being merged.

Features of this PR:
- Multi tenancy support for Adapters (undertow, tomcat, AS7)
- Two examples, one a "complete" solution with self registration and business backend, and one being the simplest possible solution for multi tenancy, including README.md files with instructions. If the "complete" is too complex for an example, I can send another PR without it.  
- Integration tests (no unit tests, as there doesn't seem to be any on the changed modules). I also confirmed that all current tests are passing.
- Documentation (Reference Guide) chapter

This branch is not compatible with the previously created branch (multi-tenant-adapter), as the individual commits have been squashed into one big commit.