[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'm not sure.  If you read the value of /socket-binding-group=standard-sockets/socket-binding=management-https/:read-attribute(name=port), then you usually get the expression ${jboss.management.https.port:9993}.  I could evaluate the expression, which should be equivalent to my System.getProperty(), but maybe not.  They could change the expression. 

But they could also change the socket binding.  They could change the name of the socket-binding or the name of the interface.  So I'm still not sure if it's more robust to rely on the address in the management model or the name of the system property?