[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.
I don't really like the idea of these being static, in particular this means that the session will survive a :reload which could potentially leak sessions as the thread that times them out will have been shut down.