Fixed check method of $USE_GET_PIP

'[[ -n "$USE_GET_PIP" ]]' always TRUE
because $USE_GET_PIP is '0' or '1'.

Change-Id: I73c4c6befe2126882ef21991b2a3fe712b2ac388
Closes-Bug: #1259824
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"
Merge "Fixed check method of $USE_GET_PIP"