Do not overrride.ssh.username

You may keep the default user (vagrant) as long
as you use Docker image "gmacario/baseimage:0.9.15b".

Leave option overrid.ssh.username="root" commented out
in case you want to test other Docker images
which may not have user "vagrant" and phusion insecure key.

Signed-off-by: Gianpaolo Macario <gmacario@gmail.com>