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.

added Enhancement label

added Frontend label

mentioned in commit 98442bcd

mentioned in merge request !457 (closed)

mentioned in commit edf3e029

mentioned in merge request !459 (merged)

This will be closed by !459 (merged), but it would be nice to also have a regression test for this.

closed via merge request !459 (merged)