(PUP-3697) Default key to certname for find action

If no key is supplied to the find action to indirector based faces,
default it to the certname. This way things such as `puppet facts find`
work without inventing a key to find facts for.
(PUP-3698) Make find the default action for facts

Finding facts is by far the most common action for the facts face so it
should be the default.
(PUP-3699) Render facts as yaml by default

Facts cannot be rendered as strings, so this change the default format
to yaml which looks decent on the console output so people don't have
to specify that all the time manually.