gsyncd / geo-rep: Partial support for Non-root geo-replication.

This patch enables geo-replication to be run as an unprivileged
user. As of now, this is just the partial support, but is very
close to achieve full functionality.

Current limitation
* Geo-replication executed Gluster CLI commands on the slave
  via SSH. On a non-root setup, Gluster CLI would run as an
  unprivileged user, failing to execute the command. As a
  workaround (for testing), setuid(2) Gluster CLI executable
  or use the glusterd option to accept commands by unprivileged
  CLI process. The nature of cli commands are "system::"
  commands (for key management) and remote volume info fetching.

Remote volume info fetching has been modified to use --remote-host
gluster cli option rather than ssh and remote cli execution.

Change-Id: Ica89e2ba9b7f48fd6e1c876c477d7822dc693617
BUG: 1077452
Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-on: http://review.gluster.org/7658
Tested-by: Gluster Build System <jenkins@build.gluster.com>