ubuntu/bootstrap: Patch debootstrap so that it retries downloads

It retries 10 times, which is arguably too many times, but that should be better than the current behaviour (try once).

Applies cleanly on git HEAD and debootstrap 1.0.61. I will try to upstream that modification.

Test:
 - Start installer, regularly run `sudo pkill wget` and check that the installation still proceeds.

Base case (no error, we don't have fault injection (yet)) tested in `2014-09-07_20-18-15_drinkcat_chroagh_deboostrap-retry_10`.

Fixes #1018.