Fixed inconsistency in iqn

Fixed inconsistency in iqn

Brick's TargetAdmin classes have a
default value set in __init__ for
iscsi_target_prefix.   The unit test was changing
the value returned for the internal _get_target,
but wasn't changing the CONF.iscsi_target_prefix to
match.  So the default value in the __init__ was
being used and hence the differences.

This patch sets the configuration's iscsi_target_prefix
which alters the values passed into the driver.  This
also makes the tests consistent between the iqn of the volume
at create time and delete time.

Closes-Bug: #1271249
Change-Id: Ie3308cf68adcbdff6057ea795af1299dded82e14