cluster/distribute: Add filter to support file patterns to be migrated

'gluster volume rebalance' command will be enhanced to support passing of these
options/pattern.

<pattern> is comma separated list as show below. The Precedence is from right
to left.

e.g- "*avi,*pdf:10MB,*:1KB"
     The precedence is as follows:
     migrate all files with size equal or greater than 1KB "*:1KB"
     migrate all pdf files with size equal or greater than 10MB "*pdf:10MB"
     migrate all avi files "*avi"

With this option, it is possible to choose which files to migrate.

Change-Id: I6d6d6a015bcbacf1debae2f278a2d92306fb055d
BUG: 896456
Signed-off-by: shishir gowda <sgowda@redhat.com>
Reviewed-on: http://review.gluster.org/4366
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>