refactor: write DOKKU_SCALE contents to scale property

- remove extraction of DOKKU_SCALE
- migrate to property system
- rewrite internals to use a FormationSlice instead of a map

tests/unit/ps-general.bats

@@ -55,7 +55,7 @@ EOF
   assert_output "node worker.js"
 }
 
-@test "(ps:scale) update DOKKU_SCALE from Procfile" {
+@test "(ps:scale) update formations from Procfile" {
   local TMP=$(mktemp -d "/tmp/dokku.me.XXXXX")
   trap 'popd &>/dev/null || true; rm -rf "$TMP"' INT TERM