tolerate payload is a unicode

Traceback (most recent call last):
  File "/home/hzgaott/openstack/stacktach/stacktach/worker/worker.py", line 124, in on_nova
    self._process(body, message)
  File "/home/hzgaott/openstack/stacktach/stacktach/worker/worker.py", line 88, in _process
    raw = dbapi.process_raw_data(self.deployment, args, asJson)
  File "/home/hzgaott/openstack/stacktach/stacktach/dbapi.py", line 215, in process_raw_data
    values = handler(routing_key, body)
  File "/home/hzgaott/openstack/stacktach/stacktach/dbapi.py", line 20, in mail_and_save
    obj = _monitor_message(routing_key, body)
  File "/home/hzgaott/openstack/stacktach/stacktach/dbapi.py", line 43, in _monitor_message
    instance = payload.get('exception', {}).get('kwargs', {}).get('uuid')
AttributeError: 'unicode' object has no attribute 'get'
Move setup log into common.log module