Healthchecks for OpenStack services

k8s provides pod healthchecking mechanism called livenessProbe. We can leverage
the mechanism to let k8s restart services pods when we detect that a pod may
have problems connecting to supporting services (MySQL, RabbitMQ), appears down
on service list or becomes unhealthy on a service-specific health checking
mechanism.

An initial idea is to create a generic Python script that will check connection
to the database and RabbitMQ. Apart from that every service may provide their
own healthchecking script that will run service-specific command (e.g. `nova
service-list` in case of Nova service or `keystone-manage doctor` to validate
Keystone config).

As healthchecks can potentially be dangerous for some kinds of deployments, we
should make them disabled by default and make it possible to enable them on
per-service basis.

This blueprint will remain as pending until an appropriate specification is drafted and agreed upon.