Display Bootstrap's hamburger menu by default and place it on the left side
If you want to create a navbar with a three line "hamburger" on mobile devices, just follow the official Bootstrap template. If you want to move the button to the left, just add a float: left to the css or use the class "pull-left" on the button. Do note that you will need to add some margin: left to make sure there's some spacing between the button and the left border.
Bootply example here