Rename saml2_token_url to token_url

For the consisteny a property returning authentication url
should be called token_url.
This patch renames ``saml2_token_url`` to ``token_url`` in the
contrib.auth.v3.saml2.Saml2UnscopedPlugin plugin.

Change-Id: I435a118bb31338a37a29eec68b8e9ce50d163675
Merge "Rename saml2_token_url to token_url"