add missing $etherpad_db_user after adding it to private repo.
the user doesn't have actual grants yet but breaks if it doesn't exist

Change-Id: I4f95309c976a879095453c3af13054a70e80997e
Merge "add missing $etherpad_db_user after adding it to private repo. the user doesn't have actual grants yet but breaks if it doesn't exist" into production