This PR introduces dependencies between the services. Now each service
has two virtual events associated with it: 'up' (running and healthy)
and 'down' (finished or failed). These events are used to establish
correct order via conditions abstraction.
Service image unpacking was moved into 'pre' stage simplifying
`init/main.go`, service images are now closer to the code which runs the
service itself.
Step 'pre' now runs after 'wait' step, and service dependencies are now
mixed into other conditions of 'wait' step on startup.
Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>
