set mustache as default template engine for spring and scala generators.

src/main/java/io/swagger/codegen/v3/generators/DefaultCodegenConfig.java

@@ -408,7 +408,7 @@ public Mustache.Compiler processCompiler(Mustache.Compiler compiler) {
 
     @Override
     public TemplateEngine getTemplateEngine() {
-        return new HandlebarTemplateEngine(this);
+        return this.templateEngine;
     }
 
     // override with any special text escaping logic

src/main/java/io/swagger/codegen/v3/generators/java/SpringCodegen.java

@@ -729,4 +729,10 @@ public void setUseBeanValidation(boolean useBeanValidation) {
     public void setUseOptional(boolean useOptional) {
         this.useOptional = useOptional;
     }
+
+    // todo: remove this once handlebar templates for this generator are implemented
+    @Override
+    protected void setTemplateEngine() {
+        templateEngine = new MustacheTemplateEngine(this);
+    }
 }

src/main/java/io/swagger/codegen/v3/generators/scala/AbstractScalaCodegen.java

@@ -249,4 +249,10 @@ protected String stripPackageName(String input) {
         int lastIndexOfDot = input.lastIndexOf(".");
         return input.substring(lastIndexOfDot + 1);
     }
+
+    // todo: remove this once handlebar templates for this generator are implemented
+    @Override
+    protected void setTemplateEngine() {
+        templateEngine = new MustacheTemplateEngine(this);
+    }
 }