The config file gets restored so the config cache needs refreshing.
Touch .guestagent.prepare.end file after upgrade to fix the problem of
the guestagent not knowing if prepare has ran.
Write the volume mount to fstab to persist after subsequent restarts.
Change-Id: I3831de12c999ef8818e80ecdb29f1d86ff8cd5c8
Closes-bug: #1645460
Depends-On: I5c1714b7839b2736c50f2daa2f4506c4006815a1