Pin pbr to avoid sphinx autodocs issues
Pin pbr to avoid sphinx autodocs issues

* https://review.openstack.org/#/c/32046/

Change-Id: I9254c735edfb555fb97dcec281d22d7fe6293c9e

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