rpc: Reconfigure() does not work for auth-reject

Problem:
If volume is set for rpc-auth.addr.<volname>.reject with value
as "host1", ideally the NFS mount from "host1" should FAIL. It
works as expected. But when the volume is RESET, then previous
value set for auth-reject should go off, and further NFS mount
from "host1" should PASS. But it FAILs because of stale value
in dict for key "rpc-auth.addr.<volname>.reject".

It does not impact rpc-auth.addr.<volname>.allow key because,
each time NFS volfile gets generated, allow key ll have "*"
as default value. But reject key does not have default value.

FIX:
Delete the OLD value for key irrespective of anything. Add
NEW value for the key, if and only if that is SET in the
reconfigured new volfile.

Upstream review:  http://review.gluster.org/7931

Change-Id: I9d1cb37002aad978a3a59e4b45b42d881d0d20e3
BUG: 1103050
Signed-off-by: Santosh Kumar Pradhan <spradhan@redhat.com>
Reviewed-on: http://review.gluster.org/8022
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>