docker module: updates

- logDriver option, use journald for logging by default
- keep storage driver intact by default, as docker has sane defaults
- do not choose storage driver in tests, docker will choose by itself
- use dockerd binary as "docker daemon" command is deprecated and will be
  removed
- add overlay2 to list of storage drivers

nixos/tests/docker.nix

@@ -11,9 +11,6 @@ import ./make-test.nix ({ pkgs, ...} : {
       { config, pkgs, ... }:
         {
           virtualisation.docker.enable = true;
-          # FIXME: The default "devicemapper" storageDriver fails in NixOS VM
-          # tests.
-          virtualisation.docker.storageDriver = "overlay";
         };
     };