Rework logging to support importing externally

Right now barbican client is not usable when importing from other
OpenStack modules. This is due to the fact that logging under
barbican.openstack.common wants to do things like register default
CONF options. And, as these have already been registered, this will
fail with exceptions like ArgsAlreadyParsedError.

Therefore, move this client to follow the examples provided by the
other python-*client modules. Namely, import standard logging module
and move the setup into the keep client itself. Now when called from
places like Nova, we will not encounter import issues.

Additionally, this change fixes a bug in barbicanclient.common.auth
where LOG was used but not defined.