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/commands/server_test.rb

@@ -0,0 +1,23 @@
+require "test_helper"
+
+class CommandsServerTest < ActiveSupport::TestCase
+  setup do
+    @config = {
+      service: "app", image: "dhh/app", registry: { "username" => "dhh", "password" => "secret" }, servers: [ "1.1.1.1" ],
+      traefik: { "args" => { "accesslog.format" => "json", "metrics.prometheus.buckets" => "0.1,0.3,1.2,5.0" } }
+    }
+  end
+
+  test "ensure run directory" do
+    assert_equal "mkdir -p kamal", new_command.ensure_run_directory.join(" ")
+  end
+
+  test "ensure non default run directory" do
+    assert_equal "mkdir -p /var/run/kamal", new_command(run_directory: "/var/run/kamal").ensure_run_directory.join(" ")
+  end
+
+  private
+    def new_command(extra_config = {})
+      Kamal::Commands::Server.new(Kamal::Configuration.new(@config.merge(extra_config)))
+    end
+end