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/commander_test.rb

@@ -99,6 +99,11 @@ class CommanderTest < ActiveSupport::TestCase
     assert_equal [ "workers" ], @kamal.roles_on("1.1.1.3").map(&:name)
   end
 
+  test "roles_on web comes first" do
+    configure_with(:deploy_with_two_roles_one_host)
+    assert_equal [ "web", "workers" ], @kamal.roles_on("1.1.1.1").map(&:name)
+  end
+
   test "default group strategy" do
     assert_empty @kamal.boot_strategy
   end