Add option to leave individual component working directories untouched during 'gdk update'

As a Gitaly developer, I find it annoying to have my Gitaly repo/working directory messed up by gdk update. This change adds a gdk.yml option to leave Gitaly alone during gdk update.

Because the gdk update code involved is so similar, I also extended this to gitlab-shell, gitlab-pages, gitlab-workhorse and gitlab-elasticsearcy-indexer.

This is part of gitaly#2229 (closed)