Fix documentation to reference manifests/site.pp

Fixing documentation in README.md which referenced manifests/site.py rather
than the correct manifests/site.pp file

Change-Id: Idee57e351f1f4836e700067167fb3dcc4c85a733
Reviewed-on: https://review.openstack.org/19137
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins