This brings over the validation and convert_to methods from Neutron.
Additional tests were added to improve coverage, and some supporting
methods were added for these.
Renamed validators to remove underscore, so that they can be imported
into neutron/api/v2/attributes.py.
Added devref docs to help developers in creating and using validators
and converters.
Change-Id: I81394dff69b816146e521bcd3e9641761178d6fd
Implements: blueprint neutron-lib
