Fix usage of oslo.config

- make it possible to create a DBAPI instance given a config instance

- remove usage of global cfg.CONF instance (it's up to a user to pass
  a ConfigOpts instance to oslo.db)

- ensure end applications don't need to import/register/use config
  options directly - instead they pass a config instance to oslo.db
  helpers (EngineFacade.from_config() and DBAPI.from_config() class
  methods)

At the same time, usage of oslo.config remains completely optional as
we provide an API to pass those parameters programatically.

Change-Id: I1a00db7a4200ebdc96f17256ecbad430477e868f
Merge "Fix usage of oslo.config"