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/integration_test.rb

@@ -78,6 +78,11 @@ class IntegrationTest < ActiveSupport::TestCase
       latest_app_version
     end
 
+    def break_app
+      deployer_exec "./break_app.sh #{@app}", workdir: "/"
+      latest_app_version
+    end
+
     def latest_app_version
       deployer_exec("git rev-parse HEAD", capture: true)
     end
@@ -131,4 +136,16 @@ class IntegrationTest < ActiveSupport::TestCase
         puts "Tried to get the response code again and got #{app_response.code}"
       end
     end
+
+    def assert_container_running(host:, name:)
+      assert container_running?(host: host, name: name)
+    end
+
+    def assert_container_not_running(host:, name:)
+      assert_not container_running?(host: host, name: name)
+    end
+
+    def container_running?(host:, name:)
+      docker_compose("exec #{host} docker ps --filter=name=#{name} | tail -n+2", capture: true).tap { |x| p [ x, x.strip, x.strip.present? ] }.strip.present?
+    end
 end