Add glance notifications tests for ceilometer

Added ceilometer test for measurements:
image.update, image.upload, image.delete, image.download, image.serve

measurements: image, image.size are not added to tests because they 
have status 'both', these measurements are pollster and notification
ones at the same time.

Partially implements: blueprint add-basic-ceilometer-tests

Change-Id: Id049b6cb8ab3092c50f35894cf5a6bda7ff04617
Move CommandFailed exception to tempest.exceptions

When the CommandFailed exception was added it was only being used by
the CLI tests. However, it is generally useful for anything that is
using subprocess to make external calls. This patch moves it to
tempest.exceptions to make using it simpler for non-cli tests to use
the exception.

Change-Id: Ibf5f1cbbb847d32976b54c4484acfc3c0e3b4f48
Merge "Move CommandFailed exception to tempest.exceptions"
Merge "Add glance notifications tests for ceilometer"