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
Verify 'update-server-metadata' APIs attributes

This patch verifies the response attributes of 'update-server-metadata'.

The response attributes of v2 and v3 are the same:
{
    "metadata": {
        "foo": "Foo Value"
    }
}

Response code for V3 API is 201.

Partially implements blueprint nova-api-attribute-test

Change-Id: If70c4cd31ed513276d0a714edc06d4878913ec48
Merge "Remove nova v2 client in ListSnapshotImagesTest"
Merge "Verify 'update-server-metadata' APIs attributes"