hydra-slave: Enable kernel same-page merging.

This is to reduce memory footprint for VMs, taken from:

https://github.com/NixOS/nixos-org-configurations/blob/34aeaec/delft/common.nix#L84-L95

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
benteflork: Only update node if there are changes.

This uses the deployment.alwaysActivate attribute from NixOps 1.1 to
ensure that the node doesn't get updated if the output from the builder
is the same as before.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
common: Always update intel microcode.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Fix attribute names for sshKeyPairs.

The final release of NixOps 1.1 has the names corrected according to the
Nix coding style, so let's adapt this correctly.

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