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/broken_deploy_test.rb

@@ -0,0 +1,24 @@
+require_relative "integration_test"
+
+class BrokenDeployTest < IntegrationTest
+  test "deploying a bad image" do
+    @app = "app_with_roles"
+
+    kamal :envify
+
+    first_version = latest_app_version
+
+    kamal :deploy
+
+    assert_app_is_up version: first_version
+    assert_container_running host: :vm3, name: "app-workers-#{first_version}"
+
+    second_version = break_app
+
+    kamal :deploy, raise_on_error: false
+
+    assert_app_is_up version: first_version
+    assert_container_running host: :vm3, name: "app-workers-#{first_version}"
+    assert_container_not_running host: :vm3, name: "app-workers-#{second_version}"
+  end
+end