packaging: setup: validating read-only user before creation

The new implementation validates provided username before
it creates a readonly user. The user provided cannot be one
of the reserved users (postgres, engine, engine_history and
engine_reports), it cannot be empty and it should not exist
in the DB.

Change-Id: Ic404e565a765c82d42f92a211a59d06c8a59d807
Bug-Url: https://bugzilla.redhat.com/1006948
Signed-off-by: Alex Lourie <alourie@redhat.com>