Configurable Kamal directory

To avoid polluting the default SSH directory with lots of Kamal config,
we'll default to putting them in a `kamal` sub directory.

But also make the directory configurable with the `run_directory` key,
so for example you can set it as `/var/run/kamal/`

The directory is created during bootstrap or before any command that
will need to access a file.

lib/kamal/cli/server.rb

@@ -14,6 +14,10 @@ class Kamal::Cli::Server < Kamal::Cli::Base
       end
     end
 
+    on(KAMAL.hosts) do
+      execute(*KAMAL.server.ensure_run_directory)
+    end
+
     if missing.any?
       raise "Docker is not installed on #{missing.join(", ")} and can't be automatically installed without having root access and the `curl` command available. Install Docker manually: https://docs.docker.com/engine/install/"
     end