Review vault placement in configuration syntax

[jfallows]

Currently vault is a top-level property of binding, even though it is not used by most binding types.

"tls0":
{
    "type" : "tls",
    "kind": "server",
    "vault": "server",
    "options":
    {
        "keys": [ "localhost" ],
        "sni": [ "localhost" ],
        "alpn": [ "echo" ]
    }
}

Consider moving to a binding-specific option instead, giving:

"tls0":
{
    "type" : "tls",
    "kind": "server",
    "options":
    {
        "vault": "server",
        "keys": [ "localhost" ],
        "sni": [ "localhost" ],
        "alpn": [ "echo" ]
    }
}