Add tweakable to command module parts allowing you to change root.

[tntristan12]

I've suggested this before, but it's never seemed more doable now that tweaks have officially been added to the game. Currently, while you can change the root part by isolating the command pod you want to switch, deleting/replacing it, then building the rest of the rocket around it, that is a lot of effort to go through if you've built a sufficiently complicated rocket around it. Furthermore, and more importantly, it might not always be possible to reconstruct the same rocket from a different root because the nodes you want to use aren't available or some such. I've run into situations myself where I could build a rocket using one command module as a root, but when trying to rebuild the same design from a different module incorporated into the design... Couldn't do it!