add way to know when entire application is up

[charlierudolph]

For local end to end testing, we need a way to know that all the services are up and running.

One possibility here is to have every service expose a port and a health check. Then we could create a new cli exo run status which wait for all health checks to respond with success.

This could be purely local to start. I don't know if we need it on remote as that means every service would need to use the update-route53 binary

@hugobho @alexdavid thoughts?

[charlierudolph]

Possibly have each service output "EXOSPHERE_ONLINE" when the service is ready.