Validators method one_of should take in account If condition too

Valid template where resources property values are "encapsulated" in an `If` condition return validation errors.

As `If` condition for a property value can be used nearly everywhere, should be better to globally change `one_of` method in validators.py to include If condition too.

Ex:
`if properties.get(property) not in conditionals:` should became:
`if properties.get(property) not in conditionals and not isinstance(properties.get(property), If):`

(there is one little problem importing If, we get a circular dep problem, so this solution need a local import or a little refactoring).

Regards, Alberto.