[CDH5] Authorize SHOW ROLES statements and support SHOW CURRENT ROLES

This patch adds the necessary changes required to authorize SHOW ROLES statements.
This is not as easy as it could be because the Sentry Service doesn't currently
expose the metadata for who is/isn't authorized to execute these statements. To authorize
the statements, we need to first make an RPC to the Sentry Service (via the
Catalog Server) and then only proceed with the SHOW statement if the check succeeds.
We should consider revisiting this approach in the future when more metadata is available
from Sentry.

Additionally, this patch adds support for SHOW CURRENT ROLES which shows all roles
that are currently granted to the current user.

Change-Id: Ia01c20d58ab081f49a85566075836d8c6e25dbd4
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/4367
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: jenkins