Enhancements for CE-322 and workaround for CM-3812

Remove any stale Apt cache files (See CM-3812) which can cause ZTP to fail.
Redirect all script output to a log file (/var/log/autoprovision) so that the
user has some visibility into what's happening (or what's gone wrong)
Do not exit on error; log the message but continue. At the worst this gives
some useful log output you can use to debug any issues.
Merge pull request #9 from CumulusNetworks/ce-322

Enhancements for CE-322 and workaround for CM-3812