Only use iscsi_helper config option if using LVMISCSIDriver

_get_iscsi_properties checks for iscsi_helper == 'tgtadm',
but this is run for drivers other than LVMISCSIDriver.

Only change target_lun based on iscsi_helper if using
LVMISCSIDriver/ThinLVMVolumeDriver.

Change-Id: I8c63f8d4097682c875bbfdfade665fdacbcf8e22
Merge "Only use iscsi_helper config option if using LVMISCSIDriver"