Convert test_image_utils tests to mock

The tests for cinder/image/image_utils.py mainly use mox. This patch
replaces the tests in cinder/tests/test_image_utils.py and adds
additional tests that use mock.

This commit removes the following trivial functions that were only used
once within image_utils and don't appear to provide any tangible
benefit:
    - file_exist
    - rename_file

Change-Id: If6c6e7370ac253f332ed532c0ca3956624267442
Merge "Convert test_image_utils tests to mock"