Add debugging support to installer

Add better debugging support to the installer.  Prompt
the user in stage1, and then in stage2, check the arguments
passed, and create a flag file if debugging is enabled.  The flag
file is checked by the postinstall script, and can be used elsewhere.
Force trusty to use ifupdown networking

As of trusty, d-i defaults to using networkmanager, which doesn't
work with any of our cluster assumptions.  Force ifupdown in
the preseed.
Update the apt-file cache; fix whitespace

Force an update of the apt-file cache on cluster (Trac: #1490)
so that users don't get prompted for it on first login.

Also, remove some stray whitespace at the end of the file