Separated get images positive tests from smoke

* Updated setUpClass() in test_get_images file to create server and
* images using ComputeIntegrationFixture and to use that in smoke test.
* Created new file test_get_images_positive to move 2 positive tests from
* test_get_images file:
- test_get_images_using_marker_pagination
- test_get_images_using_limit
* Removed unused imports

Change-Id: I4c66def5b974c32ed74d29d1b74a4c8f2f2e675e
Merge "Separated get images positive tests from smoke"