Reset ec2 image cache between S3 tests.

We recently introduced EC2 image caching in 2865a76.

On some python 2.6 distros I'm now seeing nova.tests.image.test_s3
failures caused by test ordering issues. Tests appear to reuse
IDs and this can cause issue if we don't clear the cache in tearDown.

Fixes LP Bug #1158935.

Change-Id: Ifeedaf7a784b787de95d2f36394216cf7c94c5f4
Merge "Reset ec2 image cache between S3 tests."