nixos/tests/taskserver: Add test for manual config

This subtest actually serves two purposes:

  1. Test manual PKI configuration
  2. Test changing of configuration files

In order to only test manual PKI configuration it would have been enough
to just add another server with a manual config.

But as the switch from automatic PKI config to manual config is probably
one of the most fundamental changes in configuration, so it serves
*very* well to also check whether changes in the NixOS configuration
actually have an impact in the real system.

So instead of adding another server, we now create a dummy "newServer"
machine, which is the new configuration for "server" and use
switch-to-configuration to switch "server" to the config of "newServer".

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