US3018 - Adjusts cancel methods in base plugin classes to exit immediately

https://rally1.rallydev.com/#/7675694122d/detail/userstory/23288232616

This PR adjusts three methods in the base plugin classes provided by platform: cancel_publish_group, cancel_publish_repo, and cancel_sync_repo. They used to raise NotImplementedError exceptions, but we are changing the default behavior to exit immediately. I also added tests because there were none for those methods. I've added a pretty verbose release note about this also.