Implement healthcheck on docker mode

[josejulio]

When starting the containers (cassandra, services, agent) at the same time, they sometimes fail to start.

I suspect the reason is that every service is trying to start at once and it my box lacks enough resources to properly start them up at once(more often when I have something else running).

I have to workaround this by starting services one by one. e.g. docker-compose up hawkular-cassandra wait till everything is running and then the next service (hawkular-services),wait till is ready and then hawkular-agent...

Maybe by implementing a healthcheck it would make it easier to start them automatically without issues.