Remove UUIDs in centos.json

Remove UUIDs in centos.json

Removed the remaining UUIDs from the centos.json file, making it easier
to reuse this file. This does require a recent version of the packer
tool. Updated the README accordingly and also added the --env option in
the packer alias, so that the OS_CLOUD variable is set when running
packer in the container. Updated rapid.vms with the security group name
used in the centos.json file.

Change-Id: I4f6815a3250811e336c14857e0ec5e7793a77acf
Signed-off-by: Luc Provoost <luc.provoost@intel.com>