Removal of legacy internal SSH key
Before #144 (closed), we were actually pushing internally in SSH to the auxiliary Git repository. Now that we aren't, it's better to get rid of that SSH key (registered for the root
GitLab user).
This SSH key is different on earch instance, was generated at first startup. The source installs never had it.
This was mentioned in #144 (closed), but it'll clearer now to have a specific issue.
The corresponding rake task has been included in Heptapod 0.8.0rc2, I tested it successfully before release, but it still failed while upgrading Octobus instances. Looks to be a registration line missing.