Replace FLAGS with cfg.CONF in scheduler

Replace all the FLAGS with cfg.CONF in cinder/scheduler
Large commit "https://review.openstack.org/31172" was split into several parts
Use the common pattern: CONF = cfg.CONF

Change-Id: If293d36a9e1552f7af497edf947e82458488bf9c
Fixes: bug #1182037
Merge "Replace FLAGS with cfg.CONF in scheduler"