Provide a way to run configure-commands again for workspaced elements

Now that #209 (closed) is closed, we only run configure-commands only once for workspaced elements. It it possible that the user might want to run the configure-commands again in certain cases. This is currently possible by playing the workspace close/workspace open --no-checkout game but it will be nicer to do this in one step.

As originally mentioned in !126 (comment 46286435), one way to solve it is by providing a soft reset functionality - bst workspace reset --soft. This option would simply unset the prepare flag.