Add OAuth credentials for authenticating-proxy in development VM
Add OAuth credentials for authenticating-proxy in development VM

The authenticating-proxy app is needed to protect the draft stack. Other
apps which need OAuth credentials have dummy values like these  added in
this way for the development VM which means it's possible to use the
Signon app's `make_oauth_work_in_dev` script to update the apps in
Signon with the values from the `OAUTH_ID` & `OAUTH_SECRET` env vars for
each app. Thus it makes sense to follow the same pattern for the
authenticating-proxy app.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet