API for dynamic lookup attributes

Currently we look up nodes using only BMC and MAC addresses. We should allow users to add more to this list. Initial proposal was to create a new plugin interface, but it may be an overkill for now.

The following JSON structure will be used for attributes:

 {
   "field": "JSON path to a field in data",
   "node_field": "path to field in ironic database",
   "op": "operation"
 }

Here "op" is the same as in rules API.
New API collection /v1/lookup/attributes will be created to store lookup attributes.