use 'exc_info=True' instead of import traceback

We need log traceback message sometime, but people who ignore 'exc_info'
argument would import traceback for getting traceback message. This work
is already done by 'exc_info' in logging module. For example:

logger.error('msg', exc_info=True)
                                  # exc_info evaluate as true and log
                                      message from sys.exc_info()
logger.debug('msg', exc_info=(type, value, traceback))
                                  # exc_info evaluate as an exception
                                    tuple and log message from this
                                    tuple
logger.exception('msg')
                                  # exception add exc_info=1 automatically

Change-Id: I9e1caf05fcf06bb977597076ebe278b593d70bf4
Merge "use 'exc_info=True' instead of import traceback"