Unify Hydra's NixOS module with the one used for hydra.nixos.org

In particular, the queue runner and web server now run under different UIDs.

Created by  Eelco Dolstra  on July 1, 2015
TTZ26BJQKIASUK7MT6KGQOIOOHEYWNV4UASO2ZQWGK4ADGK6SR6QC
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