nixos/hardware: Add option to enable KSM.

This is essentially what's been done for the official NixOS build slaves
and I'm using it as well for a few of my machines and my own Hydra
slaves.

Here's the same implementation from the Delft server configurations:

https://github.com/NixOS/nixos-org-configurations/blob/f47c2fc7f82cc8aa4ea8047487cbb819d6829fe2/delft/common.nix#L91-L101



Signed-off-by: aszlig <aszlig@redmoonstudios.org>