CSIT-359: adapt vpp download script for usage with 1609 branch too

- script downloads the latest vpp release from Nexus repository of the branch
  based on data in VPP_REPO_URL file stored in csit directory;
  default repo is fd.io.master.ubuntu.trusty.main

Change-Id: I254801d74fe2ef7f5e87e2c64f5619bef0709944
Signed-off-by: Jan Gelety <jgelety@cisco.com>