Remove self.op.file_storage_dir isabs check

As the manpage says, and the code does, self.op.file_storage_dir is an
additional relative path under the cluster file storage dir. As such it
should not be absolute.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>