Add VARIANT as a standard value for /etc/os-release

Some distributions (such as Fedora) are using the VARIANT field to
indicate to select packages which of several default configurations
they should be using. For example, VARIANT=Server provides a
different default firewall configuration (blocking basically
everything but SSH and the management console) whereas
VARIANT=Workstation opens many other ports for application
compatibility.

By adding this patch to the manual pages, we can standardize on a
cross-distribution mechanism for accomplishing this.

Fedora implementation details are available at
https://fedoraproject.org/wiki/Packaging:Per-Product_Configuration

(David: drop double paranthesis)