Move all files on the host under a common directory

This will make running kamal remove simpler, we can just clean up that
directory.

lib/kamal/commands/server.rb

@@ -1,5 +1,5 @@
 class Kamal::Commands::Server < Kamal::Commands::Base
-  def ensure_run_directory
-    [ :mkdir, "-p", config.run_directory ]
+  def ensure_service_directory
+    [ :mkdir, "-p", config.service_directory ]
   end
 end