Add driver filter and evaluator for scheduler

Add driver filter and evaluator for scheduler

This patch adds a new filter for the cinder scheduler that
can interpret two new properties provided by backends,
'filter_function' and 'goodness_function'.  A driver can rely
on cinder.conf entries to define these properties for a backend
or the driver can generate them some other way.  An evaluator is
used by the filter to parse the properties.  The 'goodness_function'
property  is used to weigh qualified backends in case multiple ones
pass the filter. More details can be found in the spec:
https://review.openstack.org/#/c/129330/

Implements: blueprint filtering-weighing-with-driver-supplied-functions
DocImpact:  New optional backend properties in cinder.conf.
            New filter and weigher available for scheduler.
Change-Id: I38408ab49b6ed869c1faae746ee64a3bae86be58