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.

This allows us to refer to a group of checks using a single handle.

This approach should make it easier to add new checks without having to
write lots of YAML and doing things against Ansible (e.g.
ignore_errors).

A single action plugin determines what checks to run per each host,
including arguments to the check. A check is implemented as a class with
a run method, with the same signature as an action plugin and module,
and is normally backed by a regular Ansible module.

Each check is implemented as a separate Python file. This allows whoever
adds a new check to focus solely in a single Python module, and
potentially an Ansible module within library/ too.

All checks are automatically loaded, and only active checks that are
requested by the playbook get executed.

It turned out that the playbook
`playbooks/byo/openshift-preflight/check.yml` would only work under a
certain `ansible.cfg` in which `roles/` was added to `roles_path`.

It was the case with the example config prior to
b804e70cdd0bc8601bfc87fcf3e34043223828ee.

Trying to improve the name, `init` needs to be loaded before calling other
subroles.

We don't make `init` a dependency of `common`, `masters` and `nodes` to
avoid running the relatively slow `openshift_facts` multiple times.

Note: on a simple example run of ansible-playbook against a single
docker-based host, I saw the execution time jump from 7s to 17s. That's
unfortunate, but it is probably better to reuse openshift_facts, than to
come up with new variables.

Because that's the main playbook directory in use.