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

cinder/volume/targets/iscsi.py

@@ -17,6 +17,7 @@ from oslo_log import log as logging
 
 from cinder import exception
 from cinder.i18n import _, _LI, _LW, _LE
+from cinder.openstack.common import versionutils
 from cinder import utils
 from cinder.volume.targets import driver
 from cinder.volume import utils as vutils
@@ -42,6 +43,13 @@ class ISCSITarget(driver.Target):
         self.protocol = 'iSCSI'
         self.volumes_dir = self.configuration.safe_get('volumes_dir')
 
+        # If any of the deprecated options are set, we'll warn the operator.
+        msg = _LW("The option %s has been deprecated and no longer has "
+                  "any effect. It will be removed in the Liberty release.")
+        for opt in ('iscsi_num_targets', 'iser_num_targets'):
+            if self.configuration.safe_get(opt) is not None:
+                versionutils.report_deprecated_feature(LOG, msg, opt)
+
     def _get_iscsi_properties(self, volume, multipath=False):
         """Gets iscsi configuration