14324: Remove context from interface, use Stop() instead

14324: Remove context from interface, use Stop() instead

Remove 'Context' passed into methods of the InstanceSet interface,
instead the AzureInstanceSet creates its own cancellable background
context.  The Stop() method now cancels the context and waits for any
outstanding cloud driver work to complete before returning.

* Fix test
* Document how to run individual test cases against real cloud
* ManageBlobs runs periodically in the background
* Logging cleaned up

Arvados-DCO-1.1-Signed-off-by: Peter Amstutz <pamstutz@veritasgenetics.com>