Add prefix to images created in *ImagesNegativeTest tests

Currently images created in *ImagesNegativeTest classes doesn't
use tempest prefix, just "test". This makes it hard to identify
tempest-created resources when performing cleanup. Adding prefix
to resolve the above mentioned issue.

Change-Id: Ied08c62b02e273edd5aa8a10e037eebb1f84ef85
1 file changed