Devices endpoint does not support filtering by variables

[Thomas Maddox]

Currently /v1/devices does not afford for one to filter the list server-side for only those matching a variables query.

This was brought up especially here: https://review.openstack.org/#/c/443941 and we need to support this, as the devices endpoint is an extremely useful use-case for this JSON Path-like searching.

Also, let's audit what all's missing from /v1/devices compared to /v1/hosts and address here.