(#2848) Rename certdnsnames to match new behaviour.

As part of the update to SSL the behaviour of the certdnsnames options
changed sufficiently that it would be terribly confusing to retain it.

Instead, modify the setting to warn that it is ignored, and add a new setting
to set the default subjectAltName value for bootstrapping a master
certificate.

This retains the one really useful part of the feature, without the risk that
someone will accidentally use the old name and receive a nasty surprise.

It should also draw more attention to formerly insecure configurations.

Signed-off-by: Daniel Pittman <daniel@puppetlabs.com>