Modules import nova.config for two reasons right now - firstly, to
reference nova.config.CONF and, secondly, if they use one of the
options defined in nova.config.
Often modules import nova.openstack.common.cfg and nova.config
which is a bit pointless since they could just use cfg.CONF if
they just want to nova.config in order to reference CONF.
Let's just use cfg.CONF everywhere and we can explicitly state
where we actually require options defined in nova.config.
Change-Id: Ie4184a74e3e78c99658becb18dce1c2087e450bb
94d87bce21