Description of problem:
When running the `openstack overcloud upgrade run --tags system_upgrade` command, it it fails after Leapp upgrades from RHEL 7 to 8, then that command can no longer be run again. It fails at an Ansible task saying that RHEL 8 has no 'leapp' package to install.


Version-Release number of selected component (if applicable):
RHOSP 13z12 to RHOSP 16.1 (using pre-provisioned/deployed nodes).


How reproducible:
Unknown.


Steps to Reproduce:
1. Follow the 13 to 16 FFU documentation.
    1a. https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/16.1-beta/html-single/framework_for_upgrades_13_to_16.1/index?lb_target=preview
2. Upgrade a Controller node: openstack overcloud upgrade run --tags system_upgrade --limit overcloud-controller-0


Actual results:
It fails on this task (another issue entirely; that looks like a bug in Ansible itself based on our logic in the actual role):
```
$ openstack overcloud upgrade run --tags system_upgrade --limit controller0
TASK [tripleo-bootstrap : Gather variables for each operating system] **********
Monday 15 June 2020  17:17:51 -0400 (0:00:00.116)       0:00:12.183 *********** 
fatal: [controller0]: FAILED! => {"msg": "No file was found when using first_found. Use errors='ignore' to allow this task to be skipped if no files are found"}
```

Re-run that `openstack` command and it will fail because it tries to install 'leapp' (a package only available in RHEL 7) on RHEL 8.


Expected results:
The command should be idempotent and allow re-running.


Additional info:
The FFU did end up working.

This may end up as a documentation bug as there might be a combination of tags to use to skip leapp but still run the other tasks.