Don't fail target_delete if target doesn't exist

Don't fail target_delete if target doesn't exist

There are cases seen in the Gate where a target delete is
called and an exception is raised because the target does
not exist. In the cinder target driver code we raise this
as an ISCSITargetRemoveFailed exception, but if we're asking
to delete the target and the target doesn't exist we can
probably safely move along.

This patch adds a check for this specific case and logs a warning
and continues rather than failing.  We also add a unit test to
check this case.

Change-Id: I7021cafc9ee48bb8ad54433e4482ff1d61e865ae
Closes-Bug: #1420010