Fix nil value when tracing is enabled

[mmatur]

What does this PR do?

Fix nil value when tracing is enabled

Motivation

$ docker run traefik:1.6.0-rc5 --tracing --tracing.backend=zipkin --tracing.jaeger=false
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x87f6d7]goroutine 1 [running]:
github.com/containous/traefik/middlewares/tracing/zipkin.(*Config).Setup(0x0, 0x21b1c7b, 0x7, 0xb7413b, 0xc4208d2000, 0xc4208d4000, 0x0, 0x0, 0xc4205ef578)
    /go/src/github.com/containous/traefik/middlewares/tracing/zipkin/zipkin.go:23 +0x37
github.com/containous/traefik/middlewares/tracing.(*Tracing).Setup(0xc4203d8910)
    /go/src/github.com/containous/traefik/middlewares/tracing/tracing.go:54 +0x111
github.com/containous/traefik/server.NewServer(0xc4201d44f0, 0x0, 0x100, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0xc4203d8910, ...)
    /go/src/github.com/containous/traefik/server/server.go:114 +0x58e
main.runCmd(0xc4201f8000, 0x0, 0x0)
    /go/src/github.com/containous/traefik/cmd/traefik/traefik.go:180 +0x330
main.main.func1(0x1f0c4e0, 0xc420526660)
    /go/src/github.com/containous/traefik/cmd/traefik/traefik.go:54 +0x42
github.com/containous/traefik/vendor/github.com/containous/staert.(*Staert).Run(0xc420128360, 0xc4201f8000, 0xc420526660)
    /go/src/github.com/containous/traefik/vendor/github.com/containous/staert/staert.go:84 +0x2e
main.main()
    /go/src/github.com/containous/traefik/cmd/traefik/traefik.go:142 +0x1b19

More

• Added/updated tests

[ldez]

LGTM

[juliens]

LGTM

[nmengin]

LGTM 👍