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.

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.
ubuntu/bootstrap: mv -f
installer/ubuntu/bootstrap: Check if patch applies cleanly

Fallback on original version if it fails.
Merge pull request #1026 from drinkcat/deboostrap-retry

ubuntu/bootstrap: Patch debootstrap so that it retries downloads