Fix race condition on simultaneous clean

[tailhook]

Looks like this:

$ vagga _clean --unused --at-least 1day
ERROR:vagga::wrapper::clean: Error cleaning up: Can't remove file "/root/app/some/file.py": No such file or directory (os error 2)
ERROR: Build failed: exit status 1

Basically, we lock container directories only when there is a lock file. And we create lock file only for .tmp.container-name directories, not for final containers, because they are kinda useless for containers, except perhaps for deletion.