openqa: improve how we do the git config thing
    
    The background to this is
    https://bugzilla.redhat.com/show_bug.cgi?id=2073414 , in response
    to which git was changed to die if a user runs git commands
    on a repo which it doesn't own. In openQA, the test directory
    is a git repo and openQA itself likes to run git commands on it,
    but this is often going to be as a different user than the owner
    of the directory. In fact on the worker hosts, the user that owns
    the directory (geekotest on the server box) doesn't even exist.
    
    This just sets the config by copying a file in place rather than
    running a git command (which is hard to get to be idempotent) and
    uses `/etc/gitconfig` so we don't wind up with a file in the
    _openqa-worker user's home directory, which is meant to be empty.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>