Feat maintain five instances running concurrently

Try to limit a maximum of 5 instances running on the OpenStack. Closes #53 (closed)

Decided to keep different num of images(30) since it takes less resources.

.gitlab-ci.yml

@@ -141,6 +141,11 @@ deploy_knightly:
         openstack server add floating ip $NEW_INSTANCE_ID $CAMPUS_IP
         openstack server add floating ip $NEW_INSTANCE_ID $CHEAHA_IP
       fi
+    - |
+      SERVER_TO_BE_DELETE=($(openstack server list --name ood-knightly --sort-column Image --sort-descending -f value -c ID | sed -n $(($NUM_SERVER_TO_KEEP+1))',$p'))
+      for svr in $SERVER_TO_BE_DELETE; do
+        openstack server delete ${svr}
+      done
   only:
     - schedules