Disable real HTTP connections with WebMock

In addition to disabling HTTP requests while testing, [WebMock](https://github.com/bblimke/webmock) also tells us what requests are being made during a test run, e.g.

```
Failures:

  1) Hiera::Backend::Aws::RDS#lookup returns all database instances if no tags are provided
     Failure/Error: expect(rds.lookup("rds_instances", scope)).to eq [
     WebMock::NetConnectNotAllowedError:
       Real HTTP connections are disabled. Unregistered request: POST https://rds.eu-west-1.amazonaws.com/ with body
...
```