Detach servers (remove from group without removing in Nova)

This adds the "detach" feature to the API.

- I used the term `purge` to mean "also kill the machine in Nova". I started with `delete`, but it seemed really awkward to have a DELETE request that is documented as deleting a server and then has a `delete` query argument...
- I spent a good chunk of time cleaning up the tests and fixing docstrings. It would be useful to give those a close (review) inspection to make sure I didn't screw it up.
OK, but let's make sure that doesn't hurt the October deadline.

While patching in general is nice, this does have the benefit that it makes it very easy to check what the default was. IIUC, using a test-only Request would mean a few extra tests: false vs true vs unset (default).