Add script for setting expiry and manipulating redis keys.

This is needed for dealing with sandbox key value store Redis keys.
We've more or less standardised on using `TestFoo` for classes that contain tests to run and `FooTestCase` for base test classes. Aside from convention, py.test (which we aren't using here, but are using in vumi-go) treats any class starting with `Test` as a collection of tests.

Maybe this should be `StubbedConfigHolder` instead? I'd also be happier if we were replacing less of its functionality.