Make delete_volume in volumes_client.py use **params

I99f9910f75aef095f2bbb37680ee584e52bf82da introduced a new
kwarg to the delete_volume() method. But the correct way to
do this is to introduce a generic **params argument, so avoid
changing the signature of the method too often.

Change-Id: Ifb6731fd893a32bea6c7832236bb26a2eacaf56c

releasenotes/notes/add-kwargs-to-delete-vol-of-vol-client-1ecde75beb62933c.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    The ``delete_volume`` method of the ``VolumesClient`` class
+    now has an additional ``**params`` argument that enables passing
+    additional information in the query string of the HTTP request.
+

tempest/lib/services/volume/v2/volumes_client.py

@@ -118,11 +118,16 @@ class VolumesClient(rest_client.RestClient):
         self.expected_success(200, resp.status)
         return rest_client.ResponseBody(resp, body)
 
-    def delete_volume(self, volume_id, cascade=False):
-        """Deletes the Specified Volume."""
+    def delete_volume(self, volume_id, **params):
+        """Deletes the Specified Volume.
+
+        For a full list of available parameters, please refer to the official
+        API reference:
+        https://developer.openstack.org/api-ref/block-storage/v2/#delete-volume
+        """
         url = 'volumes/%s' % volume_id
-        if cascade:
-            url += '?cascade=True'
+        if params:
+            url += '?%s' % urllib.urlencode(params)
         resp, body = self.delete(url)
         self.expected_success(202, resp.status)
         return rest_client.ResponseBody(resp, body)