[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.
As I say above it should only be parseAuthentication that will need a method for version 3, the content of security realm is not actually affected so should not need duplication, the only change being the addition of a switch statement to select the correct parseAuthentication.