Use gitlab-shell authorized_keys truncation. Fix issue with authorized_keys stored in different location

These fixes will allow a restore of gitlab when the backups and
repositories directories are in non-standard locations (ie sub-dirs
of gitlabhq).  Also allows the restore to be run from script
overriding the need of a user to confirm the rebuild of the
authorized_keys file.

Remove hardcoded refernce to gitlab-shell home. so that gitlab can be installed on any unix account other than git