glusterd/snapshot: Print correct error message on cli for snapshot operation performed on a cluster with op-version less than 30600.

Currently we get error message as  on cli 'Another transaction is in progress
Please try again after sometime' when a snapshot operation is performed
on a cluster with op-version less than 30600.
We need to print the correct error message in this case.

Change-Id: I5f144428d928393c3796bde96ce6e3a40fca8141
BUG: 1145068
Signed-off-by: Vijaikumar M <vmallika@redhat.com>
Reviewed-on: http://review.gluster.org/8371
Reviewed-by: Avra Sengupta <asengupt@redhat.com>
Reviewed-by: Sachin Pandit <spandit@redhat.com>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8796
doc : man page for snapshot commands.

This patch also contains few modifications in admin documentation.

Change-Id: I7bc2a88e6cbcfe81dcfafc2956f5b7c5524b0f0b
BUG: 1145084
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8357
Reviewed-by: Avra Sengupta <asengupt@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/8797
Reviewed-by: Vijaikumar Mallikarjuna <vmallika@redhat.com>