Rename "file" to "disk_file"

Broke this out into a separate commit since adding five characters
caused a few lines to get re-wrapped, adding more diffs.

See John's comment from Patch Set #3 for the motivation for this
change in review https://review.openstack.org/27149.

Change-Id: I1edf2cb468dcda0b781569161ada6e4016c1141c
Signed-off-by: Peter Portante <peter.portante@redhat.com>