Separated delete image positive tests from smoke

* Added setUpClass() to test_delete_image file to create server and
* image using ComputeIntegrationFixture and to use that in smoke test.
* Created new file test_delete_image_positive to move positive tests
* from test_delete_image.

Change-Id: I17338a9e311bce122ad29c654005a4ace8132993
Merge "Separated delete image positive tests from smoke"