Update openstack/common/periodic_task

The following commits are in this update:

1771a77 Adjust import order according to PEP8 imports rule
4bfb7a2 Apply six for metaclass
0ea069e Removed calls to locals()

Change-Id: Ied20ce735af79b52e7674a3dfe3b097b84b61533
Merge "Update openstack/common/periodic_task"