elk7: init at 7.0.0

This adds the following new packages:

+ elasticsearch7
+ elasticsearch7-oss
+ logstash7
+ logstash7-oss
+ kibana7
+ kibana7-oss
+ filebeat7
+ heartbeat7
+ metricbeat7
+ packetbeat7
+ journalbeat7

The default major version of the ELK stack stays at 6. We should
probably set it to 7 in a next commit.

nixos/modules/services/search/kibana.nix

@@ -5,6 +5,9 @@ with lib;
 let
   cfg = config.services.kibana;
 
+  ge7 = builtins.compareVersions cfg.package.version "7" >= 0;
+  lt6_6 = builtins.compareVersions cfg.package.version "6.6" < 0;
+
   cfgFile = pkgs.writeText "kibana.json" (builtins.toJSON (
     (filterAttrsRecursive (n: v: v != null) ({
       server.host = cfg.listenAddress;
@@ -16,6 +19,7 @@ let
       kibana.defaultAppId = cfg.defaultAppId;
 
       elasticsearch.url = cfg.elasticsearch.url;
+      elasticsearch.hosts = cfg.elasticsearch.hosts;
       elasticsearch.username = cfg.elasticsearch.username;
       elasticsearch.password = cfg.elasticsearch.password;
 
@@ -67,9 +71,30 @@ in {
 
     elasticsearch = {
       url = mkOption {
-        description = "Elasticsearch url";
-        default = "http://localhost:9200";
-        type = types.str;
+        description = ''
+          Elasticsearch url.
+
+          Defaults to <literal>"http://localhost:9200"</literal>.
+
+          Don't set this when using Kibana >= 7.0.0 because it will result in a
+          configuration error. Use <option>services.kibana.elasticsearch.hosts</option>
+          instead.
+        '';
+        default = null;
+        type = types.nullOr types.str;
+      };
+
+      hosts = mkOption {
+        description = ''
+          The URLs of the Elasticsearch instances to use for all your queries.
+          All nodes listed here must be on the same cluster.
+
+          Defaults to <literal>[ "http://localhost:9200" ]</literal>.
+
+          This option is only valid when using kibana >= 6.6.
+        '';
+        default = null;
+        type = types.nullOr (types.listOf types.str);
       };
 
       username = mkOption {
@@ -143,6 +168,19 @@ in {
   };
 
   config = mkIf (cfg.enable) {
+    assertions = [
+      {
+        assertion = ge7 -> cfg.elasticsearch.url == null;
+        message =
+          "The option services.kibana.elasticsearch.url has been removed when using kibana >= 7.0.0. " +
+          "Please use option services.kibana.elasticsearch.hosts instead.";
+      }
+      {
+        assertion = lt6_6 -> cfg.elasticsearch.hosts == null;
+        message =
+          "The option services.kibana.elasticsearch.hosts is only valid for kibana >= 6.6.";
+      }
+    ];
     systemd.services.kibana = {
       description = "Kibana Service";
       wantedBy = [ "multi-user.target" ];