optional healthcheck to overwrite the Dockerfile healthcheck

[naorlivne]

Expected/Wanted Behavior

Now that nebula-orchestrator/worker#28 is ready the healthcheck should be overwritable - the default Dockerfile healthcheck command used if noting is stated but there should be the ability to overwrite it on a per app basis (as well as the healthcheck delay and number of failures required)

Actual Behavior

Only the healthcheck CMD can be used.