Specifying a custom identifier_field is considered invalid.

[rogermparent]

Describe the bug

Attempting to use the identifier_field feature introduced in 2.2.0 causes the schema error: 'collections[0]' must have a field that is a valid entry identifier

To Reproduce

1. Attempt to load Netlify CMS with the attached config.

Expected behavior

The CMS loads without throwing an error.

Applicable Versions:

• Netlify CMS version: 2.2.0
• Git provider: test repo
• OS: Linux
• Browser version: Firefox 63.0.1 / Chromium 70.0.3538.77
• Node.JS version: 11.1.0

CMS configuration

backend:
  name: test-repo

display_url: https://example.com

publish_mode: editorial_workflow
media_folder: assets/uploads

collections: # A list of collections the CMS should be able to edit
  - label: "Blog"
    name: "blog"
    folder: "_posts/blog"
    create: true
    identifier_field: name
    fields:
      - {label: "Name", name: "name", widget: "string"}
      - {label: "Publish Date", name: "date", widget: "datetime"}
      - {label: "Featured Image", name: "thumbnail", widget: "image"}
      - {label: "Body", name: "body", widget: "markdown"}

Additional context
The problem seems to lie in netlify-cms-core/src/constants/configSchema.js because manually adding "name" to the hard-coded identifier fields allows the CMS to load, but I haven't tested anything beyond that.