Impala does not map principals to lowercase, affecting Sentry authorisation

If a user authenticates as USER@DOMAIN.COM, the string 'USER' will be passed to Sentry for authorisation. However, Sentry might expect lower-case user identifiers.

Hadoop has auth_to_local rules that can accomplish this. In the short term, we should consider mapping all usernames to lower-case (controllable by a flag).