Skip empty ECS clusters instead of failing entire backend

[ghost]

Do you want to request a feature or report a bug?

bug

What did you do?

I tried to use new ECS backend feature - autoDiscoverClusters.
So I added to my command line option ecs.autodiscoverclusters=true

It generally works, but with one remark.

I had one cluster without instances.
Until I removed it, ecs backend wasnt working at all, giving me this error

"time="2017-09-22T19:01:33Z" level=error msg="Provider connection error InvalidParameterException: Container instance cannot be empty."

There wasn't even an ECS tab in traefik UI.
I believe this is a wrong behaviour, you should just skip empty clusters, not bring down whole backend.

Output of traefik version: (What version of Traefik are you using?)

1.4rc3

[jeffreykoetsier]

PR #2170

[traefiker]

Closed by #2170.