`tart clone` should not overwrite local VM with the same name

[andriygm]

Currently, tart clone will overwrite and wipe all data for a local VM with the same name if one already exists. At minimum, it should prompt the user or cancel with a warning that a VM with this name already exists. The user should be able to --overwrite to ignore this warning and continue.

[edigaryev]

This was previously raised in #822.

tl;dr: tart clone works similarly to cp(1).

Would adding an command-line flag like cp(1)'s -i work for you?

This way you can set alias cp='cp -i' in your shell and get the desired behavior.

[andriygm]

@edigaryev did not see that previous issue, my apologies! imo, this cp-like behavior would make sense if tart had separate concepts for images and the VMs launched from those images. since they're not (or maybe this is already possible and I'm just not understanding fully), I think this behavior is still pretty unintuitive given that the result is data loss of a VM. in this case I'd still argue for not overwriting by default, but something like -i would be a nice compromise if the intent is to align with the cp interface.