Replace use of `swagger` with OpenAPI

[jamietanna]

There are a few places where we're saying Swagger instead of OpenAPI.

As Swagger has been retrospectively named OpenAPI 2.0, we should try and migrate all references accordingly.

We'll need to work around this to mark the old methods as Deprecated: and delegate to the new methods, so we can slowly phase them out of use.

I've made a start on a branch, but thought I'd get an issue raised for visibility.

[zhoub]

+1