tests/bugs: Increase timeout for rebalance completion check

Change-Id: Ic299c0d7b3996f6e85f9627430efbdf3f9ea0a8f
BUG: 884455
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/4942
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>