Add possibility to use for Ironic node custom image

Change-Id: I6e719c3a1731c91b24ff079fd70579a07bf42ef1
diff --git a/tcp_tests/templates/ironic_standalone/underlay.yaml b/tcp_tests/templates/ironic_standalone/underlay.yaml
index 8d30156..4837204 100644
--- a/tcp_tests/templates/ironic_standalone/underlay.yaml
+++ b/tcp_tests/templates/ironic_standalone/underlay.yaml
@@ -84,7 +84,7 @@
               volumes:
                 - name: system
                   capacity: !os_env NODE_VOLUME_SIZE, 150
-                  source_image: !os_env IMAGE_PATH1604
+                  source_image: {{ os_env('IRONIC_IMAGE_PATH1604', os_env('IMAGE_PATH1604')) }}
                   format: qcow2
                 - name: iso  # Volume with name 'iso' will be used
                              # for store image with cloud-init metadata.