Update TPrivilege thrift struct in preperation for GRANT/REVOKE

These are some updates that are needed for upcoming GRANT/REVOKE work. It makes
the TPrivilege struct a bit easier to work with by defining a specific privilege level
and breaking out all the object names the privilege is securing.

Change-Id: Id6414a8ff04f01df5c59385dcdb889bfe21a952a
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4137
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: jenkins