Improve reliability of preseed download in initrd

Occassionally Ubuntu fails to download its
configuration and the installation fails.
This patch enables preseed scripts to attempt
10 times to download the configuration
before failing

blueprint 100-nodes-support

Change-Id: I4dcee04d221c77bbdf82ea280ac836b73654a86e
Merge "Improve reliability of preseed download in initrd" into stable/5.1