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.