Tidy up role and host commander setup

Extract Kamal::Commander::Specifics to deal with host and role setup and
ensure that primary hosts and roles always come first. This means that
in a rolling deploy we deploy to the primary ones first.

This will be important when we remove the healthcheck step as we want
to confirm the primary host can be deployed to before completing a
deployment for other roles.

By setting the hosts and roles all together in one place we can sort
the primary ones to the front without creating infinite loops.

test/commander_test.rb

@@ -119,9 +119,10 @@ class CommanderTest < ActiveSupport::TestCase
     configure_with(:deploy_primary_web_role_override)
 
     @kamal.specific_roles = [ "web_*" ]
-    assert_equal [ "web_chicago", "web_tokyo" ], @kamal.roles.map(&:name)
+    assert_equal [ "web_tokyo", "web_chicago" ], @kamal.roles.map(&:name)
     assert_equal "web_tokyo", @kamal.primary_role.name
     assert_equal "1.1.1.3", @kamal.primary_host
+    assert_equal [ "1.1.1.3", "1.1.1.4", "1.1.1.1", "1.1.1.2" ], @kamal.hosts
   end
 
   private