[DOC] Fixed deprecated config style in devstack instruction
[DOC] Fixed deprecated config style in devstack instruction

+minor improvements

Change-Id: Ia055c60efb102ef85f524bc46d10925aed003dc2

bigfootproject/sahara,zhujzhuo/Sahara,matips/iosr-2015,ekasitk/sahara,zhangjunli177/sahara,openstack/sahara,zhujzhuo/Sahara,mapr/sahara,citrix-openstack-build/sahara,crobby/sahara,esikachev/sahara-backup,bigfootproject/sahara,redhat-openstack/sahara,zhangjunli177/sahara,egafford/sahara,tellesnobrega/sahara,keedio/sahara,xme1226/sahara,xme1226/sahara,citrix-openstack-build/sahara,ekasitk/sahara,bigfootproject/sahara,mapr/sahara,esikachev/scenario,henaras/sahara,openstack/sahara,matips/iosr-2015,citrix-openstack-build/sahara,esikachev/sahara-backup,crobby/sahara,zhangjunli177/sahara,esikachev/scenario,redhat-openstack/sahara,redhat-openstack/sahara,egafford/sahara,matips/iosr-2015,mapr/sahara,esikachev/sahara-backup,ekasitk/sahara,henaras/sahara,xme1226/sahara,esikachev/scenario,keedio/sahara,henaras/sahara,tellesnobrega/sahara,crobby/sahara,keedio/sahara,zhujzhuo/Sahara