Add 'warning' as an alias to 'warn'
Add 'warning' as an alias to 'warn'

The standard Python logging system uses 'warning' and not 'warn'. To
ease compatibility with it, let's add 'warning' too.

Change-Id: I7778d7960ca7a72be007cb083e5434ede6e3fe6e

zhurongze/oslo.messaging,eayunstack/oslo.messaging,stevei101/oslo.messaging,zhurongze/oslo.messaging,ozamiatin/oslo.messaging,hkumarmk/oslo.messaging,dims/oslo.messaging,magic0704/oslo.messaging,apporc/oslo.messaging,redhat-openstack/oslo.messaging,JioCloud/oslo.messaging,isyippee/oslo.messaging,viggates/oslo.messaging,hkumarmk/oslo.messaging,stevei101/oslo.messaging,apporc/oslo.messaging,ozamiatin/oslo.messaging,isyippee/oslo.messaging,magic0704/oslo.messaging,dukhlov/oslo.messaging,redhat-openstack/oslo.messaging,dims/oslo.messaging,dukhlov/oslo.messaging