Fix up the way we do iqn variable in unit test

Fix up the way we do iqn variable in unit test

A recent bug was reported when a developer was trying to work
with the iscsi target unit tests.  There were two problems discovered,
one of which has been addressed by the dependent patch of this commit
(https://review.openstack.org/#/c/68223/) which fixed the inconsistencies
in the string we used for iqn's in our test.

This patch addresses another issue that's somewhat related and was
exposed when the developer tried to use the "self.target_name" variable
instead of hard-coding the iqn's in the tests.

The problem is that the target_name variable uses an invalid format
for the iqn based on cinder-defaults.  It neglected to use the
volume_name prefix (in this case: "volume-").

This patch modifies the self.target_name variable to be more accurate
with what we would see in real usage, it also modifies the test to just
use this variable rather than hard coding it and running the risk of
having mismatches again in the future.

Change-Id: I392d423d5c0f58b7a21ee0c477609265002faa0c