Gitaly service should not require recipes of other services to be run

Right now, Gitaly requires the creation of gitlab_shell_secret - which in turn requires gitlab-rails and gitlab-shell recipes to be run. Ideally, to spin up Gitaly only in a node, only the recipes within Gitaly cookbook along with any common shared recipes should be required. This also ensures unwanted files are not created in the node. Combined with &14466, this should result in the Gitaly cookbook (along with package cookbook) being self-sufficient to spin up a Gitaly-only node. This should also make the implementation of #6501 (closed) easier, as we can avoid enabling gitlab_rails entirely.