Allow OAUTH IdP redirect when only one provider is defined

Currently when setting up OAUTH and having an authentication like:

    <require>org.nuxeo.ecm.platform.oauth2.openid.auth.contrib</require>
    <extension target="org.nuxeo.ecm.platform.ui.web.auth.service.PluggableAuthenticationService" point="chain">
      <authenticationChain>
        <plugins> 
          <plugin>OPENID_CONNECT_AUTH</plugin>      
        </plugins>
      </authenticationChain>
    </extension>

It ends with TOO MANY REDIRECTS error.

An improvement could be to setup a default provider and have Nuxeo automatically redirect to the OAUTH IdP server.

This for example works fine in a SAML context where the SP can automatically redirect to the IdP if the IdP is first or alone in the authentication chain.