Add minimal ES template functionality. (#12103)

When loading a template without fields, create a minimal template only applying given configuration, without any default values for mappings and settings. This allows to create additional templates only defining specific values.

CHANGELOG-developer.next.asciidoc

@@ -38,3 +38,4 @@ The list below covers the major changes between 7.0.0-rc2 and master only.
 - Reduce idxmgmt.Supporter interface and rework export commands to reuse logic. {pull}11777[11777], {pull}12065[12065], {pull}12067[12067]
 - Update urllib3 version to 1.24.2 {pull}11930[11930]
 - Add libbeat/common/cleanup package. {pull}12134[12134]
+- Only Load minimal template if no fields are provided. {pull}12103[12103]

libbeat/template/load.go

@@ -175,6 +175,9 @@ func buildBody(tmpl *Template, config TemplateConfig, fields []byte) (common.Map
 	if config.Fields != "" {
 		return buildBodyFromFile(tmpl, config)
 	}
+	if fields == nil {
+		return buildMinimalTemplate(tmpl)
+	}
 	return buildBodyFromFields(tmpl, fields)
 }
 
@@ -216,6 +219,15 @@ func buildBodyFromFields(tmpl *Template, fields []byte) (common.MapStr, error) {
 	return body, nil
 }
 
+func buildMinimalTemplate(tmpl *Template) (common.MapStr, error) {
+	logp.Debug("template", "Load minimal template")
+	body, err := tmpl.LoadMinimal()
+	if err != nil {
+		return nil, fmt.Errorf("error creating mimimal template: %v", err)
+	}
+	return body, nil
+}
+
 func esVersionParams(ver common.Version) map[string]string {
 	if ver.Major == 6 && ver.Minor == 7 {
 		return map[string]string{