glusterfs: create share of specific size

Currently, glusterfs driver creates shares, subdirectories within
GlusterFS volume, ignoring the share size argument. Use GlusterFS's
ability to set hard-limit quota on subdirectories (shares) to enable
creation of shares of specific size.

Change-Id: I28b4787e0ada4f6505d929f8d417218a0dc4d0b4
Closes-Bug: #1401000