(#21427) Make report format selectable

Without making agents able to select the format to serialize reports, a newer
agent would be unable to submit reports to an older master. Although we ask
users to upgrade masters first, that is not often done in practice. In order
to allow those kinds of setups to work, this makes the report format
selectable.

Puppet 3.2.2 and later support reports as pson, so only older, insecure 
masters need to have the agent's change this new setting.