labs: fix cp on OS X
labs: fix cp on OS X

The cp command that comes with OS X does not know the -u option. Use -f
instead.

Change-Id: Ia943f2af924e9e17045c4507ab8bafd1eaf98f3c

harrisonfeng/training-guides,Nabs007/training-guides,Nabs007/training-guides,openstack/training-guides,Nabs007/training-guides,harrisonfeng/training-guides,openstack/training-guides,harrisonfeng/training-guides