Create segment and segment tar directory only when it's missing

Currently using oak-upgrade we are dealing with source directory which should naturally exist and destination directory which in most of the cases (especially for segment-tar) does not exist at all.

It's awkward to demand from oak-upgrade caller to always create such directory as the upgrade process should create everything it needs and it can simply do this on its own in majority of simple cases.

In some sophisticated cases user should be able to create that directory on her/his own and mount i.e. external filesystem to it before using oak-upgrade but that's very special case that still should work out of the box. In that case we should never demand that the path cannot exist. It should work in both cases whether directory is created or not.