Allow heat tests to use already uploaded test image

If $HEAT_FETCHED_TEST_IMAGE is set then tempest is configured to
use the image named $HEAT_FETCHED_TEST_IMAGE for any orchestration
tests which require an image.

Fallback to checking $HEAT_CREATE_TEST_IMAGE and invoking
diskimage-builder if necessary.

The intent is to use Fedora 20 as the test image for gating
since this image has heat-cfntools already installed.

Change-Id: I177ae091a641ba99fd4c618e30a39c5148ae617f
Merge "Allow heat tests to use already uploaded test image"