(#2848) rename subject-alt-name option to dns-alt-names

The name of the option for adding subjectAltName fields to the request didn't
really indicate what it actually did: we forced the names into the DNS label
space, and generally imposed a bunch of additional behaviours.

This renames the option to better match the intended use.

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