packaging: setup: validate read-only user after input

Do not output "Error: user name cannot be empty" before asking for a
username.

Change-Id: Ia740c9ca659adc4b8ac7d80baad26312db61423c
Bug-Url: https://bugzilla.redhat.com/1057280
Signed-off-by: Yedidyah Bar David <didi@redhat.com>