Tests for nova plugin

One of the things that we do on Twisted, which I'd very much like to copy to Mimic, is that instead of writing test docstrings like:

```python
"""
Test to verify that the functionality of the function :func:`function` returns the correct result value.
"""
```

instead write docstrings that just describe the property being tested:

```python
"""
:func:`function` returns the absolute value of the sum of its arguments when provided with integer arguments.
"""
```

This makes it easier to write descriptive docstrings by avoiding words like "test" and "verify" (which, in a test, are just filler), and avoiding vague but helpful-sounding words like "correct" or "accurate".

For example, this test is verifying the response code is a 202 and that the server_id is a JSON string.  We may want to verify other cases of `POST` to `/servers` in the future, and those cases will want different docstrings.