use 'exc_info=True' instead of import traceback

We need store traceback message in log sometime. Python logging module
already provide a argument to do that. For example, we could:

logger.error('msg', exc_info=True) # log current exc_info
logger.debug(e, exc_info=(type_,value_,tb)) # log exc_info passed in arg
logger.exception('msg') # exception add exc_info automaticly

There're many places using traceback, but only two places are using
traceback for loging exc_info. (some use to store into variable, some in
openstack/common leave this to oslo)

Change-Id: I96eff1e7a0437c358753e636a8a6ac860074cb25
Merge "use 'exc_info=True' instead of import traceback"