WFLY-3591 Add keycloak auth to http management endpoint

This PR adds the ability to specify <keycloak/> 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.