Update saltstack repo archive URL, centos base img

- centos: Update qcow2 URL for aarch64 image after upstream refactored
  its repository structure;
- saltstack: Update repo URLs after upstream archived Saltstack 2017.7
  release artifacts due to recently discovered CVEs (which do not affect
  us since we run Salt Master in a Docker container without exposing its
  ports to the public network);

Change-Id: I022ba0739386734eec931c35bad51805a61b964d
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>