oslo.log has graduated from oslo-incubator, and openstack.common.log
has been removed from oslo-incubator, so let's use the new one.
NOTE1:openstack.common.log registers its options at import time, but
oslo.log needs to call register_options() explicitly.
NOTE2: split unit test case in tests/objectstore/test_swift_middleware.py
to avoid duplicate cli option register exception, see NOTE1.
Change-Id: Ida30808dbe0c584919755c207ca4ee4b91963a17