netdata service: fix permissions for apps.plugin

apps.plugin requires capabilities for full process monitoring. with
1.9.0, netdata allows multiple directories to search for plugins and the
setuid directory can be specified here.

the module is backwards compatible with older configs. a test is
included that verifies data gathering for the elevated privileges. one
additional attribute is added to make configuration more generic than
including configuration in string form.

nixos/tests/netdata.nix

@@ -0,0 +1,31 @@
+# This test runs netdata and checks for data via apps.plugin
+
+import ./make-test.nix ({ pkgs, ...} : {
+  name = "netdata";
+  meta = with pkgs.stdenv.lib.maintainers; {
+    maintainers = [ cransom ];
+  };
+
+  nodes = {
+    netdata =
+      { config, pkgs, ... }:
+        {
+          environment.systemPackages = with pkgs; [ curl jq ];
+          services.netdata.enable = true;
+        };
+    };
+
+  testScript = ''
+    startAll;
+
+    $netdata->waitForUnit("netdata.service");
+    # check if netdata can read disk ops for root owned processes. 
+    # if > 0, successful. verifies both netdata working and 
+    # apps.plugin has elevated capabilities.
+    my $cmd = <<'CMD';
+    curl -s http://localhost:19999/api/v1/data\?chart=users.pwrites | \
+       jq -e '[.data[range(10)][.labels | indices("root")[0]]] | add | . > 0'
+    CMD
+    $netdata->waitUntilSucceeds($cmd);
+  '';
+})