Feature: Allow database to index git authorized_keys

[LongLiveCHIEF]

As described in the speed up ssh operations, gitlab has a feature that will allow the gitlab_shell to index authorized_keys for git+ssh connections in the database instead of the authorized_keys file. This can drastically impact the speed of ssh connections when a large number of keys or users are present.

Implementation details:

• Add store_git_keys_in_db parameter to manifests/init.pp
• create /opt/gitlab-shell/authorized_keys keys file with git:root 0650 with content shown in docs
• add manage_sshd_git_user parameter
• use augeas resource to configure sshd with appropriate command, or use sshd drop-in file for git user