[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've been rebasing with master regularly.  So that should be OK.

But this is a more likely place for bugs.  As you've commented recently, ManagementXml is kind of a mess right now.

You are right about the schema update.  I only added a single empty tag, and indeed, I forgot to add it to the XSD.

There is no reason this can't be used in domain mode.   The <keycloak/> tag only tells domain-http to use keycloak authentication as a client of the keycloak auth server.  Since the auth server doesn't run on the domain controller, you would then configure the keycloak subsystem to use an auth server running somewhere else.