Remove nova v2 client in ListSnapshotImagesTest

ListSnapshotImagesTest should only use nova v3 client for creating
snapshot as the similar test which uses nova v2 client already exist
in nova v2 images api tests(compute/images/test_list_image_filters.py).
This patch removes nova v2 client from ListSnapshotImagesTest.

Change-Id: I04e34f7cd34f760d7bc1361a3037c1f1fe1341d8
Closes-Bug: #1311712
Merge "Remove nova v2 client in ListSnapshotImagesTest"