Added json.loads(body) to delete_attachment method

Getting error (before the fix):
TypeError: cannot convert dictionary update sequence element #0 to a
sequence

Change-Id: I90345fff4fc264d1a610d743ac192e83c1898f46
Signed-off-by: Yosi Ben Shimon <ybenshim@redhat.com>

tempest/lib/services/volume/v3/attachments_client.py

@@ -31,5 +31,6 @@ class AttachmentsClient(base_client.BaseClient):
         """Delete volume attachment."""
         url = "attachments/%s" % (attachment_id)
         resp, body = self.delete(url)
+        body = json.loads(body)
         self.expected_success(200, resp.status)
         return rest_client.ResponseBody(resp, body)