remove default=None for config options
remove default=None for config options

In the cfg module default=None is set as the default value.

Change-Id: I2f4691c5692dd6c5715ca0af7077e78a1d884b7f
Closes-bug: #1323975

vganapath/rally,eonpatapon/rally,eonpatapon/rally,openstack/rally,amit0701/rally,amit0701/rally,vganapath/rally,openstack/rally,group-policy/rally,vganapath/rally,vganapath/rally,gluke77/rally,gluke77/rally,gluke77/rally,amit0701/rally,eayunstack/rally,group-policy/rally,eayunstack/rally,yeming233/rally,yeming233/rally,eayunstack/rally,openstack/rally,openstack/rally,group-policy/rally,gluke77/rally