mgmt/glusterd: handle postvalidate carefully when prevalidate fails

* Also changed the order of peers retrieval and snapshot retrieval
  upon glusterd start, so that the snapshot bricks can be properly
  resolved while cleaning up the snapshots.

Change-Id: I120704e4412a9cadb8d90a9b7969f2b4a1196bc5
BUG: 1061685
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/7494
Reviewed-by: Kaushal M <kaushal@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>