Make negative snapshot tests faster

test_delete_image_that_is_not_yet_active is the slowest test because we
wait for the instance to be ready, which times out due to a previous
snapshot action. When trying to snapshot an instance, cancel the
operation at the end of the test as part of cleanup.

Change-Id: Ib3821fd288d21e912a8320392425f063b718abbd
Merge "Make negative snapshot tests faster"