DataTable column level policy_rules doesn't work.

[Li Rita]

Version: Ocata
Problem: The function allowed(self, request) in class Column which is in horizon/tables/base.py will throw exception "TypeError: 'str' object is not callable" if you add the "policy_rules" for DataTable column.

I check the history:
In bellowing changelist, the "DataTable column level policy" was merged on Dec 9, 2016
https://github.com/openstack/horizon/commit/0744e8e5942858756e80aff6f16fd5f89dec6077

But there was a refactor for "POLICY_CHECK_FUNCTION" on Nov 25, 2016
https://github.com/openstack/horizon/commit/f5685ebe468626a983954132c3e5754e9954563b

The refactor shall also apply to "DataTable column level policy", otherwise, the exception will be throw.