The auth header type should be taken from the openID Connect capabilities

[magopian]

See #509 (review)

When authenticating using openID Connect, the headers that are sent to the kinto server are using Bearer, which is hardcoded. Instead it should use the header_type from the capabilities, eg:

  "openid":{
    "description":"OpenID connect support.",
    "url":"http://kinto.readthedocs.io/en/stable/api/1.x/authentication.html",
    "providers":[
      {
        "name":"google",
        "issuer":"https://accounts.google.com",
        "auth_path":"/openid/google/login",
        "client_id":"248628588820-707f9m6vs19jt7g41cev4iqkglu72u58.apps.googleusercontent.com",
        "header_type":"Bearer",
        "userinfo_endpoint":"https://www.googleapis.com/oauth2/v3/userinfo"
      }
    ]
  }

Fixing this might also fix the review from @leplatrem about the ugliness of splitting/concatenating strings to get the auth type and provider for openID Connect. If we can pass the full "provider capability" object, we'll be able to extract both the provider name and the header type.

[leplatrem]

The PR #646 didn't entirely fixed this. It reads the token_type from the OpenID redirection payload but not from the server capabilites metadata.

But since we have a header_type setting on the server, we should use it.
https://kinto.readthedocs.io/en/stable/configuration/settings.html#openid-connect