Make unused iscsi_num_targets, iser_num_targets configs as deprecated

Make unused iscsi_num_targets, iser_num_targets configs as deprecated

After introducing new iscsi target driver model via Change-Id:
I43190d1dac33748fe55fa00f260f32ab209be656, configuration
options of iscsi_num_targets and iser_num_targets are not
used in these new scsi targets.

This patch makes these unused configuration options as
deprecated and adds warning messages to notify future removal
of these configs to users.

Also this patch removes _ensure_iscsi_targets() which uses these
two config options from cxt scsi target because this method is
never called anywhere.

Closes-Bug: #1427904
DocImpact: Two unused config options are deprecated
Change-Id: Ic34910f4765f46e2708de7cb686e49418f486709