fixes bug 1487548
https://bugzilla.redhat.com/show_bug.cgi?id=1487548

Add playbook to run adhoc health checks or list existing checks

This is a simple mechanism to learn what health checks are available.

Note that we defer task_vars verification, so that we can compute
requested_checks and resolved_checks earlier, allowing us to list checks
even if openshift_facts has not run.

This is useful on its own, and also aids in developing/testing new
checks that are not part of any playbook.

Since the intent when running this playbook is to execute checks, opt
for a less verbose explanation on the error summary.

And document that one needs to install dependencies before running playbooks.

We are moving toward having adhoc post-install checks and so the
"preflight" designation needs to be widened.
Updated location to playbooks/byo/openshift-checks, added health check playbook, and updated README.
Also included the certificate_expiry playbooks.
Left behind symlinks and wrappers for existing checks.

To conform with the direction of the rest of the repo, the
openshift-checks playbooks are split into two directories, one under
playbooks/common with the actual invocation and one under
playbooks/byo for entrypoints that are just wrappers for the ones in common.

Because the certificate_expiry playbooks are intended not just to be
functional but to be examples that users modify, I did not split them
similarly. That could happen later after discussion but for now I just
left them whole under byo/openshift-checks/certificate_expiry.