Fix list of tasks and their ordering

- GCE provider wasn't including the GCE SetHostname task, without which
  https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=604883 was
  preventing the hostname from getting set after reboot.
- During the GCE build, one of the GCE cleaning tasks was trying to run
  an apt-get update after the build-time resolv.conf file was removed.
  Fix this ordering by moving the network.Remove* tasks to the
  system_cleaning phase as they should have been all along, and adding
  an appropriate ordering rule for the GCE cleaning task.
- Add the fallback http.debian.net mirror after, not before, our mirror.
- The puppet plugin's ApplyPuppetManifest task specified that it should
  run before the network.Remove* tasks within the system_modification
  phase. Now that those tasks have been moved to a later phase
  (system_cleaning), remove this dependency. I have no puppet manifest
  to test this change, but am including it in hopes of avoiding a
  breakage there. Hopefully someone who uses puppet can test this or at
  least confirm that it's correct.

I've confirmed that a GCE build completes and boots with this change.