Fix false negative, when HEAT_CREATE_TEST_IMAGE is unset

Fix shell variable comparision bug
Closes-Bug: #1239041

Change-Id: Ifbc8545f929eb7bbf9b85df889dfd9fa3a96b7c0
diff --git a/lib/tempest b/lib/tempest
index bc0b18d..9f41608 100644
--- a/lib/tempest
+++ b/lib/tempest
@@ -266,7 +266,7 @@
     iniset $TEMPEST_CONF boto ssh_user ${DEFAULT_INSTANCE_USER:-cirros}
 
     # Orchestration test image
-    if [ $HEAT_CREATE_TEST_IMAGE == "True" ]; then
+    if [[ "$HEAT_CREATE_TEST_IMAGE" = "True" ]]; then
         disk_image_create /usr/share/tripleo-image-elements "vm fedora heat-cfntools" "i386" "fedora-vm-heat-cfntools-tempest"
         iniset $TEMPEST_CONF orchestration image_ref "fedora-vm-heat-cfntools-tempest"
     fi