Update Driver base objects for Keystone pluggable systems to use ABCMeta metaclass

The keystone driver classes (e.g. token, provider, assignment, identity, etc) should be updated to utilize the ABCMeta (abc module) metaclass.

The drivers should be actual abstract classes, since that is what they effectively are. This functionality is used in Neutron, Ironic, and Ceilometer. This will also help to push error discovery at class instantiation time instead of during a method call at some indeterminate time in the future. This also will allow base functionality to be implemented in the driver methods that can be called via super() but not break the method overriding requirement in subclasses.

PEP describing the use of ABCs
http://www.python.org/dev/peps/pep-3119/

This implementation should utilize six to be py3k compatible.

Example:

class AbstractBaseClassTest(six.with_metaclass(abc.ABCMeta)):
    @abc.abstractmethod
    def abstract_method1(int_arg):
        # we can do something here instead of raising
        # NotImplemented()
        return (int_arg + 1)