We were invalidating the authorizer by removing the ticket handler
entirely.  This was effective in inducing us to request a new authorizer,
but in the meantime it mean that any authorizer we generated would get a
new and initialized handler with secret_id=0, which would always be
rejected by the server side with a confusing error message:

 auth: could not find secret_id=0
 cephx: verify_authorizer could not get service secret for service osd secret_id=0

Instead, simply clear the validity field.  This will still induce the auth
code to request a new secret, but will let us continue to use the old
ticket in the meantime.  The messenger code will probably continue to fail,
but the exponential backoff will kick in, and eventually the we will get a
new (hopefully more valid) ticket from the mon and be able to continue.
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>