provision/docker/healer: small improvement on function comment
provision/docker/healer: created isRunning and isHealthy helper methods

Relates to #518
provision/docker/healer: also checking if container is running on unhealthyRunningContainers

This function will be used on ContainerHealer.Heal, so the method can
only heal running containers.
provision/docker/healer: updated unhealthyRunningContainers comment to match new function name
provision/docker/healer: implementing Heal method on ContainerHealer
provision/docker/healer: registering ContainerHealer

Also stopped receiving pointer for ContainerHealer on method definition