Remove Flags usage from cinder.volume.driver

cinder.volume.driver was using a mix of FLAGS and the
new conf options for multi-backend.  The result was failure
of multi-backend configrations to set the iscsi_helper
option.

This updates the driver.py file to use the OSLO.cfg and fixes the
check for iscsi_helper check in the initialize_connection call,
which was undefined.

Fixes bug: 1169928

Change-Id: Ia1678893f32a9d08a695f055f59f7296e17d9d78
(cherry picked from commit 168b7566a51807dfd42f873c810440472aed7171)