Don't allow cluster setups without SSL by default

[ChrisSchinnerl]

When running renterd in cluster mode it is desirable to have the traffic between workers, autopilot and bus be encrypted. The following changes should be made to make this more obvious to users.

1. Unless connecting on localhost, https:// should be required.
2. A new option DisableSSL should be added
3. renterd should print a helpful message on how to disable ssl in case that's what the user wants