glusterd: check_volume_exists should query in-memory representation

... instead of consulting the on-disk data directory. There is no reason
why the on-disk is more accurate than the in-memory representation. In
fact, it is the other way around when a node is reconciling
volume/cluster configuration with the rest of the cluster.

Change-Id: I786823efdf1d0f6b9e6fcdb72d51e5227c399ce1
BUG: 1176770
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/9292
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>