Code review comment for lp:~justin-fathomdb/nova/check-subprocess-exit-code

Merged with trunk; changed error_ok to terminate_on_stderr=False because this is what I believe is wanted (this affects a shell to vgcreate/lvcreate, so this patch would probably have helped with Bug 620027).

Also fixed a unit test that was calling release_ip it didn't get from issue_ip. Though the unit test was wrong, we weren't detecting the failure. As Ewan noticed in the most recent mailing list thread, the network unit tests are unusually tolerant of being FUBAR, this is probably why - they weren't checking exit codes!