Fix `--targets` by converting the argument to an actual list instead of relying on string handling

Fixes  #147

The problem was that we were doing `xyz in abc` where `abc` is a string, so it did substring matching (ie, `"ubuntu" in "ubuntu-debootstrap"`).  Converting it to a list (so that it becomes `"ubuntu" in ["ubuntu-debootstrap"]`) makes it function properly. :smile: