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.

test/cli/traefik_test.rb

@@ -20,7 +20,7 @@ class CliTraefikTest < CliTestCase
 
   test "reboot --rolling" do
     run_command("reboot", "--rolling").tap do |output|
-      assert_match "Running docker container prune --force --filter label=org.opencontainers.image.title=Traefik on 1.1.1.1", output.lines[3]
+      assert_match "Running docker container prune --force --filter label=org.opencontainers.image.title=Traefik on 1.1.1.1", output
     end
   end