Removing redundant, possibly flaky test

test_create_image_when_server_is_building may fail is
the VM goes to ACTIVE too fast.
The test is already covered by the images whitebox tests.

Fixes bug #1180868

Change-Id: Ied4cc5f80456c617bb45f62ef91b7d31b938b654
Merge "Removing redundant, possibly flaky test"