Remove a useless test for debots_root.

This is already tested in `find_debops_project(required=True)`.
Fix issue #73: find_up() takes exactly 2 arguments.

Using find_up here is outdated anyway, find_debops_project ist the
correct way. Some other calls did match the current interface, too.
Fix bugs in `debops-task`, remaining from shell-conversation.
Merge pull request #74 from htgoebel/issue-73-debops-task

Fix #73: debops-task script traceback