external: Allow for a single flag to control all of the fetching

This gives external users the ability to disable all of our fetch flags directly
instead of having to enable each individually every time.

Additionally, this changes our mirror site to the global ftp.debian.org
redirector to reduce the load on the US servers, as well as giving us more
access to proxied connections.

Change-Id: I60eb2cd5bedaee25ff944dccc5bf9836c6099c13
4 files changed