Drop dependency on log from oslo db code

Openstack common log module registers a few oslo.config options,
which can possibly lead to unpleasant issues (DuplicateOptError)
when we split oslo.db from oslo-incubator into a separate library
and reuse both log and oslo.db in target projects.

We don't use for example, any of the special formatting supported by
ContextAdaptor so we can use logging module from python standart library
instead of common log module.

Related to blueprint oslo-db-lib

Change-Id: I6ec426274eb3b603dbbc894762edb97515a618b2