Add ceilometer compute notifications ostf tests

Refactor all ceilometer ostf tests and add tests for
ceilometer compute notifications: memory, vcpus, disk.root.size,
disk.ephemeral.size

Closes-Bug: #1312175
Partially implements: blueprint ceilometer-ostf-notification-tests

Change-Id: I76383da70062b1194a44fe8628222ba45a220e3d
Merge "Add ceilometer compute notifications ostf tests"