Set an artificial $HOME for gitaly in test

What does this MR do?

Exercise more control over Git config resolution by setting $HOME for Gitaly in test. Also changes Rugged settings in test processes via spec/spec_helper.rb.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Reports of conflicts between developer Git config and Gitlab test git config, see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/15472.

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Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

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