Remove iscsi_helper calls from base iscsi driver

Remove iscsi_helper calls from base iscsi driver

There are a number of drivers that inherit from the base
iscsi driver, using the base implementation of initialize_connection
and terminate_connection.  These drivers also don't have iscsi_helper
defined.

We've kept all iscsi_helper specific code in the tgt helpers
themselves, or in the various versions of the LVM driver
which is where they belong.

Over time however there have been some iscsi_helper specific
calls that have crept in to the base volume.driver:ISCSIDriver
class.  These were LIO specific calls and as a result when a
configuration is set with multi-backend to use lioadm as the
target helper, or even if it's not used but the default iscsi_helper
is set to lioadm the initialize_connection and terminate_connection
calls will try and access the LIO specific calls and raise because
the drivers inheriting from this class don't have target_helper
members.

This went unnoticed mostly because there's no significant LIO testing
in place and there were no distros setting LIO as the default iscsi_helper,
it appears that this has changed however.

This patch moves the LIO specific calls back where they belong
and keeps the base ISCSIDriver generic.  I've also added a test
case for this in the SolidFire driver because it's impacted by
this.

Change-Id: Ibe010b62bb2518685753dd515326e56ffcc99cea
Closes-Bug: #1400804