CSIT changes to support new Centos 7 device testing container image.

CSIT-1505

Parse {os} parameter from job and use it to separate the correct
image for each distro and the correct stable package versions.
Use two vpp device image names, one for Ubuntu and one for Centos.

Centos sut image is in https://hub.docker.com/r/snergster/csit-sut-centos
Source for Centos sut image:
https://github.com/snergfdio/multivppcache/tree/master/csit-sut-centos

Change-Id: I568c82710d730c4ed0ca25bc12cb6074e1cbe27e
Signed-off-by: Thomas F Herbert <therbert@redhat.com>