security integration fixes, cleanup + tests

- remove use GeoNodeDataAccessManager.getAdminRole and refactor to
  direct use of GeoServerRole.ADMIN_ROLE
- add AUTHENTICATED_ROLE to credentials in dbclient (at least we're
  consistent w/ default client but not sure needed)
- speed up DefaultSecurityClientTest by many times
- do not attempt to authenticate internal geoserver root user - some
  geoserver internal calls use this for other checks
- some imports cleanup