This commit adds ValidateFunc to the policy attribute so that JSON parsing
errors can be caught early. Generally, when there is a ValidateFunc set for the
attribute, one can safely assume that before any of the creation and/or update
of the existing resource would happen it would have to succeed validation. Also
adds support for new helper function which is used to normalise JSON string.
Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>