API for listing introspection statuses

It would be handy to get status for all introspection processing running or run previously. New API:

GET /v1/introspection
 Returns statuses for nodes that are/were on introspection:
 {
   "statuses": [
      ...
   ]
 }

Support usual pagination, sorting and filtering arguments as defined by https://github.com/openstack/api-wg/blob/master/guidelines/pagination_filter_sort.rst and https://review.openstack.org/#/c/190743/

Initially support filtering by "finished" metafield:
* finished = null/absent fetches all statuses
* finished = true fetches only finished
* finished = false fetches only not finished