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: #1338490
Closes-Bug: #1339105
Partially implements: blueprint ceilometer-ostf-notification-tests

Change-Id: I295b78ae9548e0da18faa4978c5bba78c91f5585
Merge "Add ceilometer compute notifications ostf tests"