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/configuration_test.rb

@@ -272,7 +272,7 @@ class ConfigurationTest < ActiveSupport::TestCase
         volume_args: [ "--volume", "/local/path:/container/path" ],
         builder: {},
         logging: [ "--log-opt", "max-size=\"10m\"" ],
-        healthcheck: { "path"=>"/up", "port"=>3000, "max_attempts" => 7, "exposed_port" => 3999, "cord" => "/tmp/kamal-cord", "log_lines" => 50 } }
+        healthcheck: { "path"=>"/up", "port"=>3000, "max_attempts" => 7, "cord" => "/tmp/kamal-cord", "log_lines" => 50 } }
 
     assert_equal expected_config, @config.to_h
   end