[#70083992] Use MongoDB APT mirror
[#70083992] Use MongoDB APT mirror

Use our own mirror of the MongoDB repo to improve our availability. This
contains all of the package versions that we need.

Like the Puppet repo, it can be flagged off if we want to temporarily revert
to upstream.

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