Catch new iscsi exception

A while back we fixed up a target create/update error
that was identified in the gating tests:
    https://review.openstack.org/#/c/47513/

Since then a new patch was introduced to to the update and separate
it so that it could easily be used in other places.
    https://review.openstack.org/#/c/58599/

The problem is this added a new exception "TargetUpdate" and
didn't add this to the except block that we have.

This patch changes the catch block to handle the new exception and
do the retry, thereby putting the original fix back in place.

Change-Id: I1ed1bc6f4249f99a36b1168f9fec0b6b74482acf
Closes-Bug: 1223469
Merge "Catch new iscsi exception"