Due to MCP image lacks of package, introduce APT var

Change-Id: Ieb38fa3fe9ff7d7272662f9771e63e2f77edec54
diff --git a/tcp_tests/templates/virtual-offline-pike-ovs/underlay.yaml b/tcp_tests/templates/virtual-offline-pike-ovs/underlay.yaml
index 7a59e5a..0823cb4 100644
--- a/tcp_tests/templates/virtual-offline-pike-ovs/underlay.yaml
+++ b/tcp_tests/templates/virtual-offline-pike-ovs/underlay.yaml
@@ -152,6 +152,9 @@
          - name: mcp_ubuntu_1604_image           # Pre-configured image for control plane
            source_image: !os_env MCP_IMAGE_PATH1604
            format: qcow2
+         - name: apt_cloudimage1604    # This name is used for 'backing_store' option for node volumes.
+           source_image: !os_env APT_IMAGE_PATH1604  # https://cloud-images.ubuntu.com/xenial/current/xenial-server-cloudimg-amd64-disk1.img
+           format: qcow2
 
         nodes:
           - name: {{ HOSTNAME_APT01 }}
@@ -166,7 +169,7 @@
               volumes:
                 - name: system
                   capacity: !os_env NODE_VOLUME_SIZE, 150
-                  backing_store: cloudimage1604
+                  backing_store: apt_cloudimage1604
                   format: qcow2
                 - name: iso  # Volume with name 'iso' will be used
                              # for store image with cloud-init metadata.