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/fixtures/deploy_with_two_roles_one_host.yml

@@ -0,0 +1,15 @@
+service: app
+image: dhh/app
+servers:
+  workers:
+    hosts:
+      - 1.1.1.1
+  web:
+    hosts:
+      - 1.1.1.1
+env:
+  REDIS_URL: redis://x/y
+registry:
+  server: registry.digitalocean.com
+  username: user
+  password: pw