Fix flakey TestS3ImageService bug

test_show_translates_image_state_correctly sometimes fails
on some systems. This is caused because the ec2utils cache
is reset after the test rather than before the test (but after
the images are manually added to the db in setUp). This change
resets the ec2utils cache in the correct place

Fixes bug 1174266

Change-Id: I2ff9b71c932c913ca0423b1de4c04ca7d9de59cf