Add support for ignoring signal assertion keys

[mattdodge]

Imagine you had a service that should be publishing signals like this:

{
  "status": "online",
  "timestamp": "Thu Jan 17 09:24:31 PST 2019"
}

It would be useful to assert that the published signal looked correct but getting the timestamp correct on every test run would be difficult. If we could ignore the timestamp key or allow any value there we could make easier assertions. Like this for example:

self.assert_signal_published({
  "status": "online",
  "timestamp": ANY
})

The Python Mock class provides an ANY we could use if we wanted to. It also might be desirable to have that in nested dictionaries.