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/configuration.rb

@@ -57,6 +57,10 @@ class Kamal::Configuration
     Kamal::Utils.abbreviate_version(version)
   end
 
+  def run_directory
+    raw_config.run_directory || "kamal"
+  end
+
 
   def roles
     @roles ||= role_names.collect { |role_name| Role.new(role_name, config: self) }