Set sudoers secure_path for vagrant examples

secure_path is needed to find executables under i.e.
/sbin/ when vagrant uses sudo.
This fixes the problem when using vagrant-libvirt and
sync a folder via 9p which leads currently to:

==> devstack1: mounting p9 share in guest
modprobe 9p
bash: line 2: modprobe: command not found
Merge pull request #391 from toabctl/fix-vagrant-example-sudoers

Set sudoers secure_path for vagrant examples