Use Fedora 20 for DEVSTACK_GATE_TEMPEST_HEAT_SLOW

This adds a Fedora 20 image name to
$HEAT_FETCHED_TEST_IMAGE so that tempest is configured to
use it for orchestration tests requiring an image.

Fedora 20 has heat-cfntools pre-installed, so there is no
current need to run diskimage-builder during the gate run.

A separate change to devstack/stackrc will add the actual
URL to IMAGE_URLS which corresponds to this image name.

Change-Id: Iddfc6629f7c0108cb2a19c09e719ed839989bdb5
Merge "Use Fedora 20 for DEVSTACK_GATE_TEMPEST_HEAT_SLOW"