Remove the healthcheck step

To speed up deployments, we'll remove the healthcheck step.

This adds some risk to deployments for non-web roles - if they don't
have a Docker healthcheck configured then the only check we do is if
the container is running.

If there is a bad image we might see the container running before it
exits and deploy it. Previously the healthcheck step would have avoided
this by ensuring a web container could boot and serve traffic first.

To mitigate this, we'll add a deployment barrier. Until one of the
primary role containers passes its healthcheck, we'll keep the barrier
up and avoid stopping the containers on the non-primary roles.

It the primary role container fails its healthcheck, we'll close the
barrier and shut down the new containers on the waiting roles.

We also have a new integration test to check we correctly handle a
a broken image. This highlighted that SSHKit's default runner will
stop at the first error it encounters. We'll now have a custom runner
that waits for all threads to finish allowing them to clean up.

test/integration/docker/deployer/app/config/deploy.yml

@@ -28,6 +28,7 @@ builder:
     COMMIT_SHA: <%= `git rev-parse HEAD` %>
 healthcheck:
   cmd: wget -qO- http://localhost > /dev/null || exit 1
+  max_attempts: 3
 traefik:
   args:
     accesslog: true
@@ -41,3 +42,4 @@ accessories:
     roles:
       - web
 stop_wait_time: 1
+readiness_delay: 0