Cluster setup API does not support adding nodes via SSL

When adding a node to a CouchDB 2.1 cluster using the [Cluster Setup API](http://docs.couchdb.org/en/latest/cluster/setup.html#the-cluster-setup-api), using the default SSL port 6984 with a self-signed cert, returns the following response:
{"error":"bad_request","reason":"retry_later"}

## Expected Behavior
Return value
{"ok":true}

## Current Behavior
Is clustering over a SSL port is not supported? If not, then it presents a potential security issue.

## Steps to Reproduce (for bugs)
1. Setup 2 nodes with SSL (self signed cert) using the default port 6984
2. Follow the setup steps in the Cluster API where the remote port is 6984
(NOTE: This error also appears when using Fuxton)

## Your Environment
* Version used: 2.1
* Browser Name and version: Chrome
* Operating System and version (desktop or mobile): Ubutnu

Start with the Cluster Setup API documentation and reproduce the two-node setup using port 6984 and a self-signed certificate. Compare the API behavior with the same operation through Fuxton, and consider the issue complete when adding the node returns {"ok":true} over SSL.

Written by the indexing model from the issue text.