Add ability to customize ingress class

[gianrubio]

I'm restricting my ingresses to internal and external traffic. To do this, I'm running 2 ingress with [ingress--class] (https://github.com/kubernetes/ingress/blob/6cd21f7dea61e4df694d62857d5ea31f442962ce/core/pkg/ingress/controller/launch.go#L43) option.

When I finished the setup, I realise that kube-lego start ignoring my certificates with this message below.

time="2017-03-07T14:59:21Z" level=info msg="ignoring as unsupported ingress class 'intern'" context=ingress name=kibana namespace=log

This PR add the ability to kube-lego to work with different class (default: nginx,gce).

Ex:

LEGO_SUPPORTED_INGRESS_CLASS="intern,extern,nginx,custom"

[simonswine]

Sorry there is a rebase needed. Otherwise looking good and mergeable

[gianrubio]

@simonswine done!

[simonswine]

@gianrubio thanks will have to fix master first, but then will merge right away