Add a custom config parameter to the Kibana spec.

Any YAML here ends up in `kibana.yml`.

Example usage:

```yaml
apiVersion: kibana.k8s.elastic.co/v1alpha1
kind: Kibana
metadata:
  name: kibana-sample
spec:
  version: "7.1.0"
  nodeCount: 1
  elasticsearchRef:
    name: "elasticsearch-sample"
  config:
    xpack.security.sessionTimeout: 600000
```

Closes: elastic#740

operators/config/crds/kibana_v1alpha1_kibana.yaml

@@ -49,6 +49,9 @@ spec:
           type: object
         spec:
           properties:
+            config:
+              description: Config represents Kibana configuration.
+              type: object
             elasticsearch:
               description: Elasticsearch configures how Kibana connects to Elasticsearch
               properties:

operators/pkg/apis/kibana/v1alpha1/kibana_types.go

@@ -33,6 +33,9 @@ type KibanaSpec struct {
 	// +optional
 	Elasticsearch BackendElasticsearch `json:"elasticsearch,omitempty"`
 
+	// Config represents Kibana configuration.
+	Config *commonv1alpha1.Config `json:"config,omitempty"`
+
 	// HTTP contains settings for HTTP.
 	HTTP commonv1alpha1.HTTPConfig `json:"http,omitempty"`
 

operators/pkg/controller/kibana/config/settings.go

@@ -7,6 +7,7 @@ package config
 import (
 	"path"
 
+	commonv1alpha1 "github.com/elastic/cloud-on-k8s/operators/pkg/apis/common/v1alpha1"
 	"github.com/elastic/cloud-on-k8s/operators/pkg/apis/kibana/v1alpha1"
 	"github.com/elastic/cloud-on-k8s/operators/pkg/controller/common/certificates"
 	"github.com/elastic/cloud-on-k8s/operators/pkg/controller/common/settings"
@@ -25,7 +26,15 @@ type CanonicalConfig struct {
 
 // NewConfigSettings returns the Kibana configuration settings for the given Kibana resource.
 func NewConfigSettings(client k8s.Client, kb v1alpha1.Kibana) (CanonicalConfig, error) {
-	cfg := settings.NewCanonicalConfig()
+	userConfig := kb.Spec.Config
+	if userConfig == nil {
+		userConfig = &commonv1alpha1.Config{}
+	}
+
+	cfg, err := settings.NewCanonicalConfigFrom(userConfig.Data)
+	if err != nil {
+		return CanonicalConfig{}, err
+	}
 
 	esAuthSettings, err := elasticsearchAuthSettings(client, kb)
 	if err != nil {

operators/pkg/controller/kibana/config/settings_test.go

@@ -0,0 +1,91 @@
+// Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+// or more contributor license agreements. Licensed under the Elastic License;
+// you may not use this file except in compliance with the Elastic License.
+
+package config
+
+import (
+	"testing"
+
+	commonv1alpha1 "github.com/elastic/cloud-on-k8s/operators/pkg/apis/common/v1alpha1"
+	"github.com/elastic/cloud-on-k8s/operators/pkg/apis/kibana/v1alpha1"
+	"github.com/elastic/cloud-on-k8s/operators/pkg/utils/k8s"
+	uyaml "github.com/elastic/go-ucfg/yaml"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+var defaultConfig = []byte(`
+elasticsearch:
+  hosts:
+  - ""
+  ssl:
+    certificateAuthorities: /usr/share/kibana/config/elasticsearch-certs/ca.crt
+    verificationMode: certificate
+server:
+  host: "0"
+  name: ""
+xpack:
+  monitoring:
+    ui:
+      container:
+        elasticsearch:
+          enabled: true
+`)
+
+func TestNewConfigSettings(t *testing.T) {
+	type args struct {
+		client k8s.Client
+		kb     v1alpha1.Kibana
+	}
+	tests := []struct {
+		name    string
+		args    args
+		want    []byte
+		wantErr bool
+	}{
+		{
+			name: "default config",
+			args: args{
+				kb: v1alpha1.Kibana{},
+			},
+			want: defaultConfig,
+		},
+		{
+			name: "with user config",
+			args: args{
+				kb: v1alpha1.Kibana{
+					Spec: v1alpha1.KibanaSpec{
+						Config: &commonv1alpha1.Config{
+							Data: map[string]interface{}{
+								"foo": "bar",
+							},
+						},
+					},
+				},
+			},
+			want: append(defaultConfig, []byte(`foo: bar`)...),
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := NewConfigSettings(tt.args.client, tt.args.kb)
+			if tt.wantErr {
+				require.NotNil(t, err)
+			}
+			require.NoError(t, err)
+
+			// convert "got" into something comparable
+			var gotCfg map[string]interface{}
+			require.NoError(t, got.Unpack(&gotCfg))
+
+			// convert "want" into something comparable
+			cfg, err := uyaml.NewConfig(tt.want, commonv1alpha1.CfgOptions...)
+			require.NoError(t, err)
+			var wantCfg map[string]interface{}
+			require.NoError(t, cfg.Unpack(&wantCfg))
+
+			assert.Equal(t, wantCfg, gotCfg)
+		})
+	}
+}