Use Private GitHub repos

[mathieu-benoit]

• https://cloud.google.com/anthos-config-management/docs/how-to/installing-config-sync#git-creds-secret
• https://cli.github.com/manual/gh_repo_deploy-key_add

[mathieu-benoit]

Done only with Config Controller's root-sync repo, fix with 6ad2bf7. The other repos don't have sensitive info.