websso should compare remote_id_attribute to remote_id of IdP

When using the websso feature in keystone, the identity provider is looked up based on the value of the 'remote_id_attribute' environment variable provided by the SAML assertion (or claim in the case of OpenID Connect). Logic would dictate that the 'remote_id_attribute' value is searched for against the 'remote_id' field in the backend where identity providers are stored. This is not the case. Here is an example from my test environment:

When hitting http://rdo.rdodom.test:5000/v3/auth/OS-FEDERATION/websso/saml2?origin=http%3A//rdo.rdodom.test:

  {"error": {"message": "Could not find Identity Provider: https://ipa.rdodom.test/idp/saml2/metadata", "code": 404, "title": "Not Found"}}

This message contains the value of my 'remote_id_attribute' from the assertion. This value is 'https://ipa.rdodom.test/idp/saml2/metadata'. The identity provider looks like this:

[root@rdo ~(keystone_v3_admin)]$ openstack identity provider show ipsilon
+-------------+--------------------------------------------+
| Field | Value |
+-------------+--------------------------------------------+
| description | None |
| enabled | True |
| id | ipsilon |
| remote_id | https://ipa.rdodom.test/idp/saml2/metadata |
+-------------+--------------------------------------------+

You can see that the remote_id matches the value that my assertion contains.

My keystone.conf has this:

  remote_id_attribute = MELLON_IDP
  trusted_dashboard = http://rdo.rdodom.test

Keystone is currently looking up the identity provider by trying to match the value from the 'remote_id_attribute' to the 'id' field of the identity provider in keystone. This seems wrong.