Make gitlab-shell a dependency of Gitaly

Gitlab-ce calls gitlab-shell/bin/gitlab-projects which manipulates Git projects on disk. The fastest way to migrate this code is to assume gitlab-shell is installed on the Gitaly server (which is true in Omnibus) and to implement RPC's in Go that shell out to gitlab-projects.

Before we can do that we need to make gitlab-shell a formal dependency of Gitaly.

• add a configuration option in config.toml !267 (merged)

[gitlab-shell]
dir = '/home/git/gitlab-shell'

package config

type Config struct {
  GitlabShell GitlabShell `toml:"gitlab-shell"`
}

type GitlabShell struct {
  Dir string `toml:"string"`
}

• make sure gitlab-shell.dir is set in GDK
• make sure gitlab-shell.dir is set automatically on first source install in gitlab-ce https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13306
• make sure gitlab-shell.dir is set in gitlab-ce CI config.toml https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13306
• make sure the relevant gitlab-ce update guide mentions the change https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13309
• make sure omnibus renders gitlab-shell.dir in config.toml (path will be hard-coded to /opt/gitlab/embedded/service/gitlab-shell) omnibus-gitlab!1833 (merged)