The OpenID Connect grant type is something implicit to our plugins,
so there is no need to pass it as a keyword argument, we can define
this as a class argument. Moreover, it makes no sense to pass a
grant_type that is different to the grant type that is being
implemented. This change deprecates the keyword argument (showing a
warning to the users if they're passing it) and checking if the
grant_type is different to the one defined by the plugin itself.
Change-Id: I4fffc2c651487d3240bbc8458ad728b06b0ffa64
885aff00ac