glusterd: harden 'volume start' staging to check for brick dirs' presence

PROBLEM:

When the brick directory of a volume is absent on any of the servers,
AND an attempt is made to start the volume, commit fails ONLY on the
node where the brick dir is absent, leading to a split-brain like
situation.

FIX:

Harden 'volume start' to check for the presence of brick directories
at the time of staging, thereby preventing commit failure.

Change-Id: I67faeb9afbd3aa76f08645924462db126bf7a977
BUG: 889996
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: http://review.gluster.org/4365
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>