Fix wrong exception reference

iscsi.py should catch cinder.exception.NotFound instead of
cinder.brick.exception.NotFound.

Change-Id: I77134683712e9706619a7f7fe82caed42ede5887
Closes-Bug: #1300136
(cherry picked from commit 115b8447618073d3383bbd612ebd2fc68c8c5d28)