Base validation framework implemented

* validate(schema, *validators) - method validates request body using the
  specified jsonschema and then validate request body using validators
  (callables);

* check_exists(get_func, *id_prop, **get_args) - method checks that object
  is available using specifed function and magic props for defining id-arg
  mapping for calling function;

* ApiValidator added w/ support of uuid type;

* unit tests for ApiValidator has been added.

Partially implements blueprint savanna-rest-api-1-0-validation

Change-Id: Ieb2b9c46ab58e7ec4900016f21164d07bcfba30d
Merge "Base validation framework implemented"