Update tests to use CONF.set_override

This patch updates some tests that were overriding configuration
values by means other than CONF.set_override. It also removes any
left over references to the glusterfs_disk_util config option.

Closes-bug: 1284293
Closes-bug: 1284300

Change-Id: Id2d14cc3ff77322d017033f3558aed9cd637589b
Merge "Update tests to use CONF.set_override"