Switch terminology of 'impersonated user' to 'delegated user'

This is to help ensure naming is consistent across the platform and
also avoid confusion with HS2 "impersonation" which is something very
different.

Change-Id: I48c1b76dff75b92b11ddc7aab0eb9a3a5d20e489
Reviewed-on: http://gerrit.ent.cloudera.com:8080/3315
Reviewed-by: Lenni Kuff <lskuff@cloudera.com>
Tested-by: jenkins