Open id connect

@cvasilak I've added the method login to allow OpenID Connect login. would you mind reviewing?

If you want to test it with real app, see:
https://github.com/corinnekrych/aerogear-ios-cookbook-1/tree/AGIOS-287
with this realm config:
https://github.com/corinnekrych/aerogear-backend-cookbook-1/tree/AGIOS-287.SharedShoot.openIDConnect

Note: I haven't finish demo implementation yet. Need to talk to Andrea to make it nicer and display shoot images before submitting PR.
actually the main difference with oauth2 flow is that when you go to authz endpoint you add more "scopes", one being mandatory is openid. See:
http://openid.net/specs/openid-connect-core-1_0.html#AuthRequest
Now we could use whether or not the userinfo endpoint is filled in but keycloak does not provide a user info endpoint, besides from GoogleConfig, the user expressed wether it's a OpenID connect flow with a boolean. To keep consistent i use both bool and url as optional.