Only use iscsi_helper config option if using ISCSIDriver

_get_iscsi_properties checks for iscsi_helper == 'tgtadm', but
this is run for drivers other than ISCSIDriver.  This means that
when using the Nexenta driver, the target_lun is changed when it
shouldn't be.

Only change target_lun based on iscsi_helper if using ISCSIDriver.

Change-Id: I104c1b45f37917ea9f41855c17405f63c4ebcc39
Merge "Only use iscsi_helper config option if using ISCSIDriver" into stable/folsom