Add release note for "network delete" command for nova network

Change-Id: Idb8a24465e447e90315c0f614ad91bd7eabd6878
partial-Bug: 1543672

Author: Tang Chen

releasenotes/notes/bug-1543672-bad2fc4c6c8f3125.yaml

@@ -0,0 +1,4 @@
+---
+features:
+  - Command ``network delete`` is now available for nova network.
+    [Bug `1543672 <https://bugs.launchpad.net/python-openstackclient/+bug/1543672>`_]