Update git submodules

Update git submodules

* Update docs/submodules/samplevnf from branch 'master'
  - 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>