MT#5389 Use piuparts from wheezy-backports

Should fix piupart builds against lenny, see #699028

Thanks Victor for spotting and reporting
Use ftp.ie.debian.org as default Debian mirrpr

Getting failures like:

| E: Failed to fetch http://http.debian.net/debian/pool/main/s/spandsp/libspandsp2_0.0.6~pre21-2_amd64.deb: Error reading from server. Remote end closed connection [IP: 46.4.205.44 80]

during package builds is annoying and since we use
eu-west1 as our EC2 region let's try to keep close.