Standard bootstrap main menu with dropdown?
Your "standard" Bootstrap subtheme launches with your "standard" Bootstrap Nav Bar. For most intents and purposes this is the main-menu... links added to the main menu at admin/structure/menu/manage/main-menu appear there ... 

Oddly the navbar isn't available to configure from the blocks page as you might expect nevertheless ... there are some navbar options available under admin/appearance/your-bootstrap-subtheme/settings.