Remove duplicated test for caching image

The test test_cache_image_success is (bad) duplicate of the
test test_cache_image

The test is trying to retrieve a real url and saving it into
a block device. Mock several calls to avoid this behavior and
and assert that the cached image is properly saved.

Change-Id: Ie2ff5931cc77e5cdd1621c652d73a62fed0b69a6
Partial-Bug: #1326849
Merge "Remove duplicated test for caching image"