Make use of oslo.log for Mistral

Currently, most projects of OpenStack have already use oslo.log modules for their internal log functionality. So for Mistral, we should also make use of oslo.log, for 3 reasons listed below:

* oslo.log library provides standardized configuration for all openstack projects. It also provides custom formatters, handlers and support for context specific logging (like resource id's etc). It's a more general way for logging in OpenStack.

* As for Mistral already being an OpenStack service under OpenStack namespace,we need to use libraries and technologies adopted by OpenStack
community like oslo.messaging for RPC communications(which is already done in Mistral, great!), and others. oslo.log is indeed another important part of that.

* now, some log behavior is not very graceful in Mistral, just like what it does here https://github.com/stackforge/mistral/blob/master/mistral/utils/wf_trace.py#L25. I expect to make it better, using oslo.log capabilities.