Remove Brick's iscsi dependency on cinder

This patch removes Brick's iscsi/iscsi.py's
dependence on cinder.  Brick can't contain
cinder dependencies as it will be used in nova.

Fixes Bug #1209012

Change-Id: I046b5a53baecce81ba184cb7f30ec450f4738dae
Merge "Remove Brick's iscsi dependency on cinder"