Add soft_write_failure to puppetdb.conf

This supports a recently added feature to PuppetDB.
Add params to support soft_write_failure option as per Ken Barber

I can rebase these commits once the code is debugged and to your
liking.
Document puppetdb_soft_write_failure in README
Merge pull request #89 from ghoneycutt/soft_write_failure

Add soft_write_failure to puppetdb.conf