Add compute service tags to ListSnapshotImagesTest

The tests in ListSnapshotImagesTest all require nova because they
create a server in setUpClass and then snapshot it so they can test
filtered image lists. This commit adds the appropriate service tag so
the tests are identified as using nova.

Partially implements bp add-service-tags

Change-Id: Idec3110e8c1edfc34b77bf08bb927a8c10596914
Merge "Add compute service tags to ListSnapshotImagesTest"