(#2848) Eliminate redundant master_dns_alt_names.

Now that dns_alt_names applies to local CSR generation, there is no need
for a special configuration option only applied to a master certificate.

Eliminating that option allows us to simplify the overall model, and provide
more uniform access to the subjectAltName setting when required.

Documentation about the option is also updated.

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