Add a in container exec based health check.

Why is this different than TCP/HTTP health checker registration?  I don't like this piece randomly hanging out in space, apart from its siblings.  I guess it's because it needs the kubelet pointer?

What if we add a level of indirection and make health-check registrations more dynamic?  Call health.InitHealthCheckers(k) from here, and have that encapsulate the registration, which can later become proper plugins or something.