geo-rep: retire old style ssh setup

Users are still using geo-rep with the old, deprecated, insecure, unsupported
ssh setup. Not their fault -- the implementation of the new method had the
following charasteristics:
- old method is possible, but with default settings it's not working
- it can be made operational by fiddling with "remote-gsyncd" tunable
- with default setting, an unhelpful, actually misleading error message is
  produced
- the UI gave no hint to the changes in the ssh setup

http://review.gluster.org/4392 tried to fix these; what it accomplished was
unrestricted support to the bad practice (by making the default old setup
operational).

From this on:
- we disable the old method by reserving the "remote-gsyncd" tunable
- if the old method is attempted, give a hint what to do

Change-Id: Icade94725d8d8d2d4c89cab992d4226351637b86
BUG: 895656
Signed-off-by: Csaba Henk <csaba@redhat.com>
Reviewed-on: http://review.gluster.org/4602
Reviewed-by: Venky Shankar <vshankar@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>