[feature] Endpoint for listing revision validations with details

This patch set adds a new endpoint to the Validations API which allows
for listing all validations for a given revision with details.
The response body for GET /api/v1.0/{revision_id}/validations/detail
looks like:

  ---
  count: 1
  next: null
  prev: null
  results:
    - name: promenade-site-validation
      url: https://deckhand/api/v1.0/revisions/4/validations/promenade-site-validation/entries/0
      status: failure
      createdAt: 2017-07-16T02:03Z
      expiresAfter: null
      expiresAt: null
      errors:
        - documents:
            - schema: promenade/Node/v1
              name: node-document-name
            - schema: promenade/Masters/v1
              name: kubernetes-masters
          message: Node has master role, but not included in cluster masters list.

Note that the Validations API in general is currently missing fields
like url (as well as next and prev references) which will be included
in a follow up.

This will enable Shipyard to avoid performing a quadratic number
of API look ups when querying Deckhand's Validations API: [0].

The policy enforced for this endpoint is deckhand:list_validations.

APIImpact
DocImpact

[0] 06b5e82ea8/shipyard_airflow/control/configdocs/deckhand_client.py (L265)

Change-Id: I827e5f47bffb23fa16ee5c8a705058034633baed

releasenotes/notes/list-validations-details-endpoint-c3f18963b1372e40.yaml

@@ -0,0 +1,8 @@
+---
+features:
+  - |
+    Adds a new endpoint to the Deckhand Validations API,
+    GET /api/v1.0/{revision_id}/validations/detail, which allows for the
+    possibility of listing all validations for a revision with details.
+    The response body includes all details returned by retrieving
+    validation details for a specific validation entry.