Whitelist log for deprecated configuration.

[ldez]

What does this PR do?

Whitelist log for deprecated configuration.

Motivation

goroutine 26 [running]:
runtime/debug.Stack(0x772c4e, 0xc0000a8230, 0x2627259)
       /usr/local/go/src/runtime/debug/stack.go:24 +0xae
runtime/debug.PrintStack()
       /usr/local/go/src/runtime/debug/stack.go:16 +0x29
github.com/containous/traefik/safe.defaultRecoverGoroutine(0x21c38e0, 0x449ac60)
       /go/src/github.com/containous/traefik/safe/routine.go:148 +0x8c
github.com/containous/traefik/safe.GoWithRecover.func1.1(0x26d4850)
       /go/src/github.com/containous/traefik/safe/routine.go:139 +0x5e
panic(0x21c38e0, 0x449ac60)
       /usr/local/go/src/runtime/panic.go:513 +0x1c7
github.com/containous/traefik/server.(*Server).buildMiddlewares(0xc0004d0580, 0xc000b52e19, 0x1d, 0xc000483340, 0xc000b46390, 0xc000b34b10, 0x5, 0xc00020e720, 0x26029b0
, 0x4, ...)
       /go/src/github.com/containous/traefik/server/server_middlewares.go:58 +0x1834
github.com/containous/traefik/server.(*Server).loadFrontendConfig(0xc0004d0580, 0x26029b0, 0x4, 0xc000b52e19, 0x1d, 0xc000b463c0, 0xc0009ed110, 0xc0009ed290, 0xc00095ca
80, 0xc00095cab0, ...)
       /go/src/github.com/containous/traefik/server/server_configuration.go:168 +0xba6
github.com/containous/traefik/server.(*Server).loadConfig(0xc0004d0580, 0xc0009ed0e0, 0xc00067e260, 0x0, 0x100, 0x0, 0x0, 0x0, 0x0, 0x0, ...)
       /go/src/github.com/containous/traefik/server/server_configuration.go:97 +0x1d4
github.com/containous/traefik/server.(*Server).loadConfiguration(0xc0004d0580, 0x26029b0, 0x4, 0xc000b463c0)
       /go/src/github.com/containous/traefik/server/server_configuration.go:45 +0x239
github.com/containous/traefik/server.(*Server).listenConfigurations(0xc0004d0580, 0xc000070a80)
       /go/src/github.com/containous/traefik/server/server_configuration.go:436 +0x58
github.com/containous/traefik/server.(*Server).Start.func2(0xc000070a80)
       /go/src/github.com/containous/traefik/server/server.go:254 +0x3b
github.com/containous/traefik/safe.(*Pool).Go.func1()
       /go/src/github.com/containous/traefik/safe/routine.go:78 +0x3e
github.com/containous/traefik/safe.GoWithRecover.func1(0x26d4850, 0xc0004fda40)
       /go/src/github.com/containous/traefik/safe/routine.go:142 +0x54
created by github.com/containous/traefik/safe.GoWithRecover
       /go/src/github.com/containous/traefik/safe/routine.go:136 +0x50

[juliens]

LGTM

[nmengin]

LGTM

[dtomcej]

LGTM