ipareplica: Limit the number of concurrent executions for the setup of CA

There is a pull request and also a proposal for ansible be able to limit the
number of concurrent executions for a single task:

- https://github.com/ansible/proposals/issues/129
- https://github.com/ansible/ansible/pull/42528

The keyword is currently named max_concurrent, but might be renamed later
on. If the keyword is present, but not supported by ansible, it will be
simply ignored. Therefore there is no issue right now with adding in here
early.