Add deployment keys to gitlab:shell:setup

For people who need access to the 'git' account via SSH (needed for Capistrano
for example) you can copy a unique SSH public key into
config/deployment_authorized_keys and run `rake gitlab:shell:setup`

This is an alternative to [Gitshell #15](https://github.com/gitlabhq/gitlab-shell/pull/15) that
should address all of @randx's concerns, specifically that if someone
steals an admin account they will not gain full ssh access.