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>