Make probes use TLS against scope.weave.works by default

[2opremio]

Also:

• Fix wss:// for websockets
• Only target scope.weave.works implicitly if no app is provided

I have tested this against the scope.weave.works (i.e. scope launch --probe-token=foo), and all connections are successful.

Fixes #777

@tomwilkie PTAL and make whatever corrections you think are necessary since I will be away for 3 weeks.

[2opremio]

For the record, while testing a probe against scope.weave.works (which still runs Scope Apps with version 0.10.0) it panics when trying to pause a container:

goroutine 46 [running]:
github.com/weaveworks/scope/xfer.(*JSONWebsocketCodec).ReadRequestHeader(0xc8202afe00, 0xc8202afe20, 0x0, 0x0)
        /go/src/github.com/weaveworks/scope/xfer/controls.go:143 +0xa7
net/rpc.(*Server).readRequestHeader(0xc8208eff40, 0x7fa207562820, 0xc8202afe00, 0x0, 0x0, 0xc8202afe20, 0x41ae00, 0x0, 0x0)
        /usr/local/go/src/net/rpc/server.go:576 +0x90
net/rpc.(*Server).readRequest(0xc8208eff40, 0x7fa207562820, 0xc8202afe00, 0xc8206fe180, 0x416f1e, 0x1e1b230, 0x0, 0x0, 0x0, 0x0, ...)
        /usr/local/go/src/net/rpc/server.go:543 +0x8b
net/rpc.(*Server).ServeCodec(0xc8208eff40, 0x7fa207562820, 0xc8202afe00)
        /usr/local/go/src/net/rpc/server.go:462 +0x8c
github.com/weaveworks/scope/xfer.(*appClient).controlConnection(0xc82011eb00, 0x4f9700, 0x0, 0x0)
        /go/src/github.com/weaveworks/scope/xfer/app_client.go:217 +0x482
github.com/weaveworks/scope/xfer.(*appClient).(github.com/weaveworks/scope/xfer.controlConnection)-fm(0xc800000008, 0x0, 0x0)
        /go/src/github.com/weaveworks/scope/xfer/app_client.go:225 +0x2e
github.com/weaveworks/scope/xfer.(*appClient).doWithBackoff(0xc82011eb00, 0x1397400, 0x8, 0xc82011bf58)
        /go/src/github.com/weaveworks/scope/xfer/app_client.go:170 +0x9e
github.com/weaveworks/scope/xfer.(*appClient).ControlConnection.func1(0xc82011eb00)
        /go/src/github.com/weaveworks/scope/xfer/app_client.go:225 +0x22d
created by github.com/weaveworks/scope/xfer.(*appClient).ControlConnection
        /go/src/github.com/weaveworks/scope/xfer/app_client.go:226 +0x35

but this is not really related to this PR since it also happens with master. I believe it's just just a backwards compatibility problem caused by ac9c011#diff-24706e06628363dd12b48d439304a70dR143 which I believe will always panic (it shouldn't though).

Pity, I thought controls were backwards compatible.

[tomwilkie]

LGTM!