add_node: Don't fail if node name != "couchdb" or "node1"

add_node: Don't fail if node name != "couchdb" or "node1"

Adding nodes to a cluster fails if the node names (the `name` of
`name@hostname` in vm.args) is different from "couchdb".
The code currently infers this name from the port: "node1" if 15984,
"node2" if 25984, "node3" if 35984, "couchdb" otherwise. No other
possibility.

This is not suited for a production set-up, where multiple servers could
have different names.

This patch fixes this problem by adding an optional "name" option to the
"add_node" command:

  POST /_cluster_setup
  {
    "action": "add_node",
    "username": "root",
    "password": "******",
    "host": "production-server.com",
    "port": 5984,
    "name": "node5"
  }

This fixes: COUCHDB-3119