gluster: Modification of logic for volume capacity calculation.

This patch modifies the logic for gluster volume capacity
computation. Earlier the volume capacity was calculated
wrt non root user. Now logic is modified to calculate
volume capacity wrt root user.

Change-Id: I044e37699d60d1eed2f95eb65189d949c5e4b594
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1129619
Signed-off-by: ndarshan <dnarayan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/31451
Reviewed-by: Sahina Bose <sabose@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>