glusterd/snapshot : Fix for snapshot delete failure

Problem : snapshot delete used to fail when executed in loop,
as there was race between process kill and umount.

Solution : Before an umount is issued check if the process
is still running, If so then issue for process termination.
Give three tries for doing umount operation

Change-Id: I7f4315e5d7d4a156dd513ec77443ead6ccd37b2e
BUG: 1090449
Signed-off-by: Vijaikumar M <vmallika@redhat.com>
Signed-off-by: Sachin Pandit <spandit@redhat.com>
Reviewed-on: http://review.gluster.org/7532
Reviewed-by: Atin Mukherjee <amukherj@redhat.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>