glusterd/snapshot: fixing glusterd quorum during snap operation

During a snapshot operation, glusterd quorum will be checked only
on transaction peers, which are selected in the begin of the
operation, and not on the entire peer list which is susceptible
for change for any peer attach operation.

Change-Id: I089e3262cb45bc1ea4a3cef48408a9039d3fbdb9
BUG: 1114403
Signed-off-by: Joseph Fernandes <josferna@redhat.com>
Reviewed-on: http://review.gluster.org/8200
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-by: Kaushal M <kaushal@redhat.com>
Tested-by: Kaushal M <kaushal@redhat.com>