elk: 6.2.4 -> 6.3.2

* The ELK stack is upgraded to 6.3.2.

* `elasticsearch6`, `logstash6` and `kibana6` now come with X-Pack which is
  a suite of additional features. These are however licensed under the unfree
  "Elastic License".

* Fortunately they also provide OSS versions which are now packaged
  under: `elasticsearch6-oss`, `logstash6-oss` and `kibana6-oss`.
  Note that the naming of the attributes is consistent with upstream.

* The test `nix-build nixos/tests/elk.nix -A ELK-6` will test the OSS
  version by default. You can also run the test on the unfree ELK using:
  `NIXPKGS_ALLOW_UNFREE=1 nix-build nixos/tests/elk.nix -A ELK-6 --arg enableUnfree true`

lib/licenses.nix

@@ -231,6 +231,12 @@ lib.mapAttrs (n: v: v // { shortName = n; }) rec {
     fullName = "Eiffel Forum License v2.0";
   };
 
+  elastic = {
+    fullName = "ELASTIC LICENSE";
+    url = https://github.com/elastic/elasticsearch/blob/master/licenses/ELASTIC-LICENSE.txt;
+    free = false;
+  };
+
   epl10 = spdx {
     spdxId = "EPL-1.0";
     fullName = "Eclipse Public License 1.0";