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.

lib/kamal/sshkit_with_ext.rb

@@ -103,3 +103,34 @@ class SSHKit::Backend::Netssh
 
   prepend LimitConcurrentStartsInstance
 end
+
+require "thread"
+
+module SSHKit
+  module Runner
+    class ParallelCompleteAll < Abstract
+      def execute
+        threads = hosts.map do |host|
+          Thread.new(host) do |h|
+            begin
+              backend(h, &block).run
+            rescue ::StandardError => e
+              e2 = SSHKit::Runner::ExecuteError.new e
+              raise e2, "Exception while executing #{host.user ? "as #{host.user}@" : "on host "}#{host}: #{e.message}"
+            end
+          end
+        end
+
+        exception = nil
+        threads.each do |t|
+          begin
+            t.join
+          rescue SSHKit::Runner::ExecuteError => e
+            exception ||= e
+          end
+        end
+        raise exception if exception
+      end
+    end
+  end
+end