Changing override.ssh.username on an existing machine breaks rsync step

[maljub01]

Changing override.ssh.username on an existing machine breaks rsync step due to an error in permissions.

Steps to reproduce:

1. Set override.ssh.username to "root"
2. vagrant up
3. Change override.ssh.username to "ubuntu"
4. vagrant provision

The output of the last step will be as follows:

There was an error when attempting to rsync a share folder.
Please inspect the error message below for more info.

Host path: ...
Guest path: /vagrant
Error: Warning: Identity file [".../.ssh/id_rsa"] not accessible: No such file or directory.
rsync: failed to set permissions on "/vagrant/...": Operation not permitted (1)
rsync error: some files could not be transferred (code 23) at /SourceCache/rsync/rsync-42/rsync/main.c(992) [sender=2.6.9]

After looking at the machine and code, it looks like this is happening because the sync_folders step changes the owner of the rsync target's guest path to the correct user, but doesn't do so recursively, which causes rsync to fail when it tries to touch anything inside /vagrant.
#98 & #171 might be related, but I'm not too sure.