CI: Fix ironic scenario image builds

Needed after I2f2bdbc9e1c6ad6da4ac7098ddd36143123c3062

Change-Id: I218a7a4245065115e5823c761704fcee9995d275

Author: mnasiadka

tests/setup_gate.sh

@@ -37,7 +37,7 @@ function prepare_images {
         GATE_IMAGES+=",^aodh,^tacker,^mistral,^redis,^barbican"
     fi
     if [[ $SCENARIO == "ironic" ]]; then
-        GATE_IMAGES+=",^dnsmasq,^ironic,^iscsid"
+        GATE_IMAGES+=",^dnsmasq,^ironic,^iscsid,^prometheus"
     fi
     if [[ $SCENARIO == "magnum" ]]; then
         GATE_IMAGES+=",^designate,^magnum,^trove"