Backoff retry for service launching

[sirsean]

Currently, the Kafka and Desdemona containers sleep for a constant number of seconds before trying to start up, to give the services they depend on time to finish launching and configuring themselves. This helps us "avoid" a startup race condition, but is obviously wrong.

We need to be able to attempt to start, and if the dependencies are not ready yet, pause and wait to try again rather than completely failing. It should have an exponential backoff to maximize reliability.

[lvh]

This would (should) also have the benefit of not making /bin/bash pid 1, and ideally having a real init for it, even if it's a dumb init a la phusion's my-init or Yelp's dumb-init.