NCBC-725: Make the ICluster interface public so that it can be mocked

This changes the name of ICOuchbaseCluster to ICluster and makes it a
public interface, making it capable of being mocked using a mocking
framework such as Moq.

Change-Id: I55f84f23441520a076e37d23bb2c1af9b4588049
Reviewed-on: http://review.couchbase.org/43587
Reviewed-by: Simon Baslé <simon@couchbase.com>
Reviewed-by: Jeffry Morris <jeffrymorris@gmail.com>
Tested-by: Jeffry Morris <jeffrymorris@gmail.com>