Jortel cancel agent requests

The agent cancellation solution includes:
 - support in gofer 0.76 (pulp.spec updated)
 - added cancel_request() method to the agent manager.  (converted to ReST docstirngs while I was there)
 - added cancel() method to the agent proxies.
 - updated the Conduit in the gofer plugin to give the handlers access to the cancelled flag in the gofer context.
 - Update consumer content (install, update & uninstall) itineraries.  Added a cancel callback that simply calls cancel() in the agent manager.

Cancel flow:
 (cancel) -> Task.cancel() -> cancel_callback() -> AgentManager.cancel_request() -> Agent.cancel() -> __gofer-magic__ -> pulpplugin.Conduit.cancelled()

Added test cases and update the agent testing mocks.

Fixed a few things I noticed while in the code.