allow overwriting of http_image in tempest.conf

the default http_image variable is an upstream cirros url in
tempest. However, in the gate this can cause flakey failures if
that has connection issues.

Ensure that we can override this from devstack-gate to be
something we are sure is going to work.

Partial fix for bug #1190623

Change-Id: Ie06fc231725c235f5ee8fd17fc87d3305bc845a3

lib/tempest

@@ -206,6 +206,13 @@ function configure_tempest() {
     iniset $TEMPEST_CONF identity alt_tenant_name $ALT_TENANT_NAME
     iniset $TEMPEST_CONF identity admin_password "$password"
 
+    # Image
+    # for the gate we want to be able to override this variable so we aren't
+    # doing an HTTP fetch over the wide internet for this test
+    if [[ ! -z "$TEMPEST_HTTP_IMAGE" ]]; then
+        iniset $TEMPEST_CONF image http_image $TEMPEST_HTTP_IMAGE
+    fi
+
     # Compute
     iniset $TEMPEST_CONF compute change_password_available False
     # Note(nati) current tempest don't create network for each tenant