Add mirror url used for validation

jenkins/jobs/ironic.yaml

@@ -243,6 +243,8 @@
 
           export DEVSTACK_LOCAL_CONFIG=""
 
+          export NODEPOOL_MIRROR_HOST="nova.clouds.archive.ubuntu.com"
+
           # Tells devstack that we're going to use real hardware
           if [ "{ironic-is-hardware}" == "1" ] ; then
             # Deploy to hardware platform