Fix docstring for affinity policy
Fix docstring for affinity policy

This patch fixes the docstring for the pre_op method in the affinity
policy implementation. The current docstring is inaccurate and
confusing.

Change-Id: I00270df24527ecb15a314831001aed9f877f38f2

openstack/senlin,openstack/senlin,stackforge/senlin,openstack/senlin,stackforge/senlin