Missing /etc/centos-release in continuous streams

[jlebon]

On downstream releases:

# cat /etc/centos-release
CentOS Linux release 7.4.1708 (Core)
# cat /etc/redhat-release
CentOS Linux release 7.4.1708 (Core)

Continuous releases:

# cat /etc/centos-release
cat: /etc/centos-release: No such file or directory
# cat /etc/redhat-release
CentOSDev release 7.%{centos_rel} (Devel)

Looks like centos-devel-atomic-host-release needs a few tweaks.