There is a analysis [1] suggested to run queries against specific
columns rather than full ORM entities to optimize the performance.
Right now, it is impossible to execute such optimization because
OVO doesn't support fetching specific column yet.
This commit introduces a new method 'get_values' in the base
neutron object class. Subclass of neutron object can leverage
this method to fetch specific field of a OVO. It supports fetching
non-synthetic fields only as syntheic fields are not directly backed
by corresponding DB table columns.
neutron-lib patch: https://review.openstack.org/#/c/619047/
[1] https://review.openstack.org/#/c/592361/
Needed-By: https://review.openstack.org/#/c/610184/
Change-Id: Ib90eae7738a5d2e4548fe9fed001d6cdaffddf3b
Partial-Implements: blueprint adopt-oslo-versioned-objects-for-db
f24f422373