Convert string types for property type system.
Convert string types for property type system.

This causes the Property type system to automatically convert any string type
argument that is passed into it to basestring, so that clients of the API can
pass in str or unicode objects.

BUG=
R=iannucci@chromium.org

Review URL: https://codereview.chromium.org/1342243004 .

shishkander/recipes-py,luci/recipes-py,luci/recipes-py,shishkander/recipes-py