Separated update image positive tests from smoke

* Added setUpClass() to test_update_image file to create server and
* image using ComputeIntegrationFixture and to use that in smoke test.
* Created new file test_update_image_positive to move 3 positive tests
* from test_update_image file:
- test_update_image_add_additional_property
- test_update_image_remove_additional_property
- test_update_image_replace_additional_property
* Removed unused imports
* Changed name of image property from "new_prop" to "user_prop"

Change-Id: I4b3e97307a7bad1c6fb6d68b05bd91a9f9ecccab
Merge "Separated update image positive tests from smoke"