Don't backup to the local filebucket by default

Puppet agent's will backup any overwritten or deleted file to its local filebucket via the default backup parameter for the file type. It defaults to puppet which means to use the filebucket named puppet which is automatically created.

Many users run into trouble because the filebucket will grow unbounded, such as PUP-1137. It has been proposed to use the tidy resource to clean up stale files, but can lead to memory issues with large numbers of file resources being created. See https://github.com/npwalker/pe_metric_curl_cron_jobs/issues/3

For this ticket we should change the default value of the bucket parameter from puppet to false, see https://github.com/puppetlabs/puppet/blob/6.1.0/lib/puppet/type/file.rb#L110, and update the file reference documentation to reflect the new behavior.