Description of problem:
FFU: openstack overcloud ffwd-upgrade run fails when nova services run on a different role than controller:

2018-06-21 15:03:56,663 p=21084 u=mistral |  TASK [Create cell0 db] *********************************************************
2018-06-21 15:03:56,745 p=21084 u=mistral |  skipping: [192.168.24.18] => {"changed": false, "skip_reason": "Conditional result was False"}
2018-06-21 15:03:59,060 p=21084 u=mistral |  fatal: [192.168.24.10]: FAILED! => {"changed": false, "msg": "unable to find /root/.my.cnf. Exception message: (2002, \"Can't connect to local MySQL server through socket '/var/lib/mysql/mysql.sock' (2)\")"}

Indeed there is no /root/.my.cnf file on this custom role(it's only con controller nodes) and this is required by the mysql_db module:

https://github.com/openstack/tripleo-heat-templates/blob/master/docker/services/nova-api.yaml#L426-L429

Version-Release number of selected component (if applicable):
openstack-tripleo-heat-templates-8.0.2-38.el7ost.noarch

How reproducible:
100%

Steps to Reproduce:
1. Deploy OSP10 with nova services running on a custom role different than controller
2. Upgrade undercloud to 11/12/13
3. Run ffwd-upgrade prepare
4. Run openstack overcloud ffwd-upgrade run 

Actual results:
Fails during the following task:
https://github.com/openstack/tripleo-heat-templates/blob/master/docker/services/nova-api.yaml#L426-L429

because there is no /root/.my.cnf file on the custom role making it impossible for the mysql_db task to run.

Expected results:
No failure.

Additional info:
Attaching sosreport.