Support advanced validation of dictionaries in the API.

This change allows to specify constraints for individual keys in
dictionaries in the specification of the API resources.

Introduces type:dict_or_none validator that allows to skip
validation of a dictionary if a None value was supplied.

Adds constraints for session persistance parameters to the API.

Disables specifying cookie_name for session persistence, if
the type is set to HTTP_COOKIE or SOURCE_IP.

Fixes: bug #1104110

Change-Id: I62068deed449967949eaba9259028c88dd6974f8