Trello-1963: access publishing-api from aws frontends
Trello-1963: access publishing-api from aws frontends

After the frontends migrate to AWS, some will still require access to
the publishing-api, which is nginx load balanced via the backend-lb, so
here we are opening up the publishing-api to be able to accept connects
from the 3 egress nat gateway IPs from aws.

@ronocg <conor.glynn@digital.cabinet-office.gov.uk>

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