core: Enable log-in for a regular user via cloud-init

Cloud-Init oVirt currently support only root user.
This patch add the ability to select any user name or use the default
username (by leaving the field blank) or using the root user (by setting
the user name as root)

Change-Id: Iebfe44ec68e8bcfae5087504bf5cd29b29aad49b
Signed-off-by: Shahar Havivi <shaharh@redhat.com>