Adding centos.json to be used with packer to generate a VM with PROX

Adding centos.json to be used with packer to generate a VM with PROX

Based on a CentOS cloud image downloadable from the web, you can now use
packer to create an image that has DPDK and PROX installed in the
proper directories. The VM is also tuned for guest VNFs.
A service is also created to deal with the proper isolated_cpu settings
for tuned. The service will also make sure uio and igb_uio are loaded.
The prox_user_data.sh is not needed any more and hence deleted.
ctrl-c of runrapid.py will now also stop prox in all the VMs so a new
test can be started.

Change-Id: I16cc59878e2d4912757f42e05f98d51dff5feb89
Signed-off-by: Luc Provoost <luc.provoost@intel.com>