GlusterFS: Remove glusterfs_disk_util option

The glusterfs_disk_util option lets one choose whether to
use df or du to calculate space usage.  This serves no real
useful purpose.  Should remove this option to simplify things.

Since this does not meaningfully impact behavior, removal of
this option shouldn't require any special deprecation efforts.

(This same change was made for the NFS driver in 5bf7b9be.)

DocImpact: Remove config option

Change-Id: I302692454b43de15688d65873a72a3dada0c67cb
Catch TypeError when new_size is None on extend

The volume_actions extend method does not catch/handle TypeError
exceptions.  The tempest volume_actions test includes a negative
test that doesn't pass in a new-size so the result is we get an
unhandled exception/trace in the log files for these runs.

Change-Id: I8b699a28e06e62126da02c3318d3129412dffa6b
Closes-Bug: #1259336
Merge "Catch TypeError when new_size is None on extend"
Merge "GlusterFS: Remove glusterfs_disk_util option"