HEAT_FETCHED_TEST_IMAGE may not be defined in some cases.

Some scripts that source stackrc use set -u, which will error
when unknown variables are accessed

Change-Id: I5af2d58475e1793dfa728b7ce9180fcbba1145e9
diff --git a/stackrc b/stackrc
index e89e64d..b129197 100644
--- a/stackrc
+++ b/stackrc
@@ -282,6 +282,7 @@
         IMAGE_URLS=${IMAGE_URLS:-"http://download.cirros-cloud.net/0.3.1/cirros-0.3.1-x86_64-uec.tar.gz"};;
 esac
 
+HEAT_FETCHED_TEST_IMAGE=${HEAT_FETCHED_TEST_IMAGE:-""}
 if [[ "$HEAT_FETCHED_TEST_IMAGE" == "Fedora-i386-20-20131211.1-sda" ]]; then
     IMAGE_URLS+=",https://dl.fedoraproject.org/pub/alt/stage/20-RC1.1/Images/i386/$HEAT_FETCHED_TEST_IMAGE.qcow2"
 fi