Update docstring for api...node.validate

Add the param to the docstring for api.controllers.v1.node.validate.
With this change, the generated webapi page [1] will include this
information. Without it, it shows:
    Validate the driver interfaces. :type node_uuid: uuid

[1] http://docs.openstack.org/developer/ironic/webapi/v1.html

Change-Id: Ia33755b7ef1473aa0d04b22c76f20b7f1a99cd6b
Merge "Update docstring for api...node.validate"