Add sync calls to all prepare scripts

Strictly speaking, we only need sync calls at the end of the
script that is directly invoked by nodepool.  But just so we
don't have to think about it too much, add them to all prepare
scripts.

Without it, clouds will happily snapshot a system without all
the data written to disk!

Closes-Bug: 1282136

Change-Id: Id7dcf6667d6e48eb5ae95fd42b09f0651eeb38f9
Merge "Add sync calls to all prepare scripts"