Wrapped vbguest.auto_update setter inside a condition

Otherwise, we would get warnings/errors when vbguest wasn't installed.
Moved the ssh-add insecure_private_key into localsetup

This was previously done on script loading, but with a brand new vagrant
installation, that insecure key is not present. By moving this code
after the first vagrant up, we are sure that the key exists (because
that key is created after the first command is ran).