Description of problem:
OSP11 -> OSP12 upgrade: major-upgrade-composable-steps-docker fails "The conditional check 'mongod_service.stat.exists' failed. The error was: error while evaluating conditional (mongod_service.stat.exists): 'mongod_service' is undefined"

Version-Release number of selected component (if applicable):
openstack-tripleo-heat-templates-7.0.0-0.20170901051303.0rc1.el7ost.noarch

How reproducible:
100%

Steps to Reproduce:
1. Deploy OSP11 with 3 controllers, 1 compute, 3 ceph storage nodes
2. Upgrade undercloud to OSP12
3. Run major upgrade composable docker steps to upgrade to containerized architecture:

timeout 100m openstack overcloud deploy \
--templates /usr/share/openstack-tripleo-heat-templates \
--libvirt-type kvm \
--ntp-server clock.redhat.com \
-e /usr/share/openstack-tripleo-heat-templates/environments/ceph-ansible/ceph-ansible.yaml \
-e /home/stack/virt/internal.yaml \
-e /usr/share/openstack-tripleo-heat-templates/environments/network-isolation.yaml \
-e /home/stack/virt/network/network-environment.yaml \
-e /home/stack/virt/hostnames.yml \
-e /home/stack/virt/debug.yaml \
-e /home/stack/virt/nodes_data.yaml \
-e /usr/share/openstack-tripleo-heat-templates/environments/docker.yaml \
-e /usr/share/openstack-tripleo-heat-templates/environments/docker-ha.yaml \
-e /usr/share/openstack-tripleo-heat-templates/environments/major-upgrade-composable-steps-docker.yaml \
-e /home/stack/ceph-ansible-env.yaml \
-e /home/stack/docker-osp12.yaml \


Actual results:
Upgrade fails on:

    TASK [Stop and disable mongodb service on upgrade] *****************************
    fatal: [localhost]: FAILED! => {"failed": true, "msg": "The conditional check 'mongod_service.stat.exists' failed. The error was: error while evaluating conditional (mongod_service.stat.exists): 'mongod_service' is undefined"}
    	to retry, use: --limit @/var/lib/heat-config/heat-config-ansible/12bd6b75-e971-4816-9e6b-94c761cd78e0_playbook.retry


Expected results:
Upgrade completes ok.

Additional info:
Note: https://review.openstack.org/#/c/501120/ was applied as a workaround for bug 1490288