Add TRole/TPrivilege structs to Thrift CatalogObjects

These are used as our internal representation of the authorization policy metadata
(as opposed to directly using the Sentry Thrift structs). Versioned/managed in the
same way as other TCatalogObjects.

Change-Id: Ia1ed9bd4e25e9072849edebcae7c2d3a7aed660d
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2545
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: jenkins