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/base.rb

@@ -79,6 +79,8 @@ module Kamal::Cli
 
         run_hook "pre-connect"
 
+        ensure_run_directory
+
         acquire_lock
 
         begin
@@ -167,5 +169,11 @@ module Kamal::Cli
       def first_invocation
         instance_variable_get("@_invocations").first
       end
+
+      def ensure_run_directory
+        on(KAMAL.hosts) do
+          execute(*KAMAL.server.ensure_run_directory)
+        end
+      end
     end
 end