Refactor build and release process

* Automatically sign and release for sonatype.
* Execute findbugs on reporting stage instead of build
  (because our jenkins fails on findbugs for some reason...)
Add support for HTTPS & Unix sockets

* Respect the DOCKER_HOST and DOCKER_CERT_PATH environment variables and
  connect to Docker instances over HTTPS or via Unix sockets.
* Remove all DOCKER_HOST setting logic.
* All the logic has been moved to github.com/spotify/docker-client.