Reduce `magic` conf attribute usage
Reduce `magic` conf attribute usage

Connection class contains `magic` attribute conf, which contains (or at
least should contain) all config options. Set these config options as
class attributes to make the code more clear

Change-Id: Ib67f148e2ecf3def37e57d6e3359bced932fbc90

isyippee/oslo.messaging,dims/oslo.messaging,zhurongze/oslo.messaging,stevei101/oslo.messaging,apporc/oslo.messaging,hkumarmk/oslo.messaging,isyippee/oslo.messaging,dims/oslo.messaging,stevei101/oslo.messaging,dukhlov/oslo.messaging,apporc/oslo.messaging,zhurongze/oslo.messaging,magic0704/oslo.messaging,ozamiatin/oslo.messaging,hkumarmk/oslo.messaging,ozamiatin/oslo.messaging,dukhlov/oslo.messaging,magic0704/oslo.messaging