Update the skeleton subsystem for keycloak to align with current practices

WFLY-16632 added a skeleton subsystem for keycloak in order to be able to migrate from the keycloak subsystem to the elytron-oidc-client subsystem.

The keycloak subsystem code was imported directly from keycloak's repo. We need to update this to align with current practices as described in the comments here:

https://github.com/wildfly/wildfly/pull/15864