nixos/dhparams: Add a defaultBitSize option

This allows to set the default bit size for all the Diffie-Hellman
parameters defined in security.dhparams.params and it's particularly
useful so that we can set it to a very low value in tests (so it doesn't
take ages to generate).

Regardless for the use in testing, this also has an impact in production
systems if the owner wants to set all of them to a different size than
2048, they don't need to set it individually for every params that are
set.

I've added a subtest to the "dhparams" NixOS test to ensure this is
working properly.

Signed-off-by: aszlig <aszlig@nix.build>