[glusterd/snapshot] snapshot create force option

Implement force option in snapshot create i.e

1) Creation of snapshot fails if the original volume
bricks are down
2) With a force option creation of snapshot will continue
even if the original volume bricks are down.

This was the fix for bugs 1089527 and 1083502

Change-Id: I8de0242adf8ee0af00db9fa8701d86fabc12e7fc
BUG: 1090042
Signed-off-by: Joseph Fernandes <josferna@redhat.com>
Reviewed-on: http://review.gluster.org/7520
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>