basessh/distgit: adjust the way ssh is configured for distgit
    
    Basically, we are now installing a small wrapper in /usr/local/bin
    which just echoes to stdout what should be in the authorized_keys
    file for that user.
    That content is generated by retrieving the ssh key from sssd via
    the command sss_ssh_authorizedkeys as well as the usual ssh way to
    restrict the action an user/key can do: command="...".
    In this case, we're setting a couple of environment variable that
    are needed later on for things to work properly as well as only
    allow the user to call the aclchecker.py script provided by pagure.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>