Add force_ssl flag to Riak Control.

Separate out authentication type and forced SSL requirements.
Specifically, a user should be able to force SSL when using no
authentication, or force non-SSL when using authentication given they
may already be on a protected network.

Upgrade notes:

Users will, if they wish to continue forcing SSL with the userlist
authentication method, configure force_ssl in the app.config file to
true.

Also, we'll need a separate PR for adding these options to riak.schema.

Open question: should we default SSL to true or false?

@seancribbs @joedevivo