Adding tool for automated cluster-merger

This is the implementation of docs/design-cluster-merger.rst. It allows
the automatic merging of one or more clusters into the invoking cluster.

While this version is tested and working it still needs some tweaking
here and there for error handling and user experience.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>