Switch from http.debian.net to ftp.debian.org

Out of 10 recent ISO builds within ~4 days 9 of them failed. :(
At least 7 of those 9 broken builds were caused by broken downloads:

| 20141023 => 07:52:28 E: Failed to fetch http://http.debian.net/debian/pool/main/libr/librsvg/librsvg2-common_2.40.4-1_amd64.deb: Error reading from server. Remote end closed connection [IP: 2001:41c8:1000:21::21:35 80]
| 20141024 => 06:47:25 W: Failed to fetch xz:/var/lib/apt/lists/partial/http.debian.net_debian_dists_testing_main_binary-amd64_Packages  Hash Sum mismatch
| 20141026 => 05:29:52 W: Failed to fetch http://http.debian.net/debian/dists/testing/main/i18n/Translation-en  Hash Sum mismatch
| 20141027 => E: Unable to find a source package for gcc-4.7
| 20141027 => E: Unable to find a source package for gcc-4.7
| 20141027 => 05:10:38 W: Failed to fetch xz:/var/lib/apt/lists/partial/http.debian.net_debian_dists_testing_main_binary-amd64_Packages  Hash Sum mismatch
| 20141027 => 09:32:37 Err http://http.debian.net/debian/ testing/main libsystemd-daemon0 amd64 215-5+b1
|             09:32:37   Error reading from server. Remote end closed connection [IP: 2001:41c8:1000:21::21:35 80]
| 20141027 => 20:29:56 E: Failed to fetch http://http.debian.net/debian/pool/main/s/systemd/libsystemd-daemon0_215-5+b1_amd64.deb: Error reading from server. Remote end closed connection [IP: 2001:41c8:1000:21::21:35 80]
| 20141027 => 22:44:01 E: Failed to fetch http://http.debian.net/debian/pool/main/libx/libxfont/libxfont1_1.4.99.901-1_amd64.deb: Error reading from server. Remote end closed connection [IP: 2001:41c8:1000:21::21:35 80]

This is a serious development and release stopper for us.
Therefore let's try switching to ftp.debian.org which has been
reported to have a useful network bandwidth and failover setup available
(thanks zobel + weasel).