[WFLY-3591] Add keycloak auth to http management endpoint

This PR adds the ability to specify &lt;keycloak/&gt; as the authentication provider for the ManagementRealm in domain-http.  It does not actually pull any Keycloak artifacts into the target build.  On its own, this PR doesn't add any functionality in wildfly-core.  However, this is needed so that Keycloak can be added to wildfly full.
This is a Keycloak thing.  There are always two ways to configure a client adapter and the Keycloak Admin UI will show you the configuration for both json and  subsystem XML.  So it could be confusing if you aren't allowed to use json.

That being said, I'd like to encourage everyone to use the subsystem's XML setup.  The automatic setup I'm doing right now uses the subsystem.  So I might come back later and get rid of the json thing unless someone comes up with a compelling use case to keep it.  Maybe I should ask on the Keycloak mailing list.