Add a configuration key to disable managing user's ssh key in pagure
    
    Some deployment may use a different system where the ssh keys are handled
    outside of pagure.
    With this configuration key you can turn off this part of pagure so it
    does not get in your way.
    
    Fixes #2465
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>