Feature request: Add ingress annotation to enable upstream "keepalive" option

[vdavidoff]

In the latest NGINX release, 0.9.0-beta.7, there is no mention of the keepalive option from the upstream module. It'd be nice to be able to configure this, presumably with an annotation on an ingress resource. I'm thinking something like ingress.kubernetes.io/upstream-keepalive: {integer} (explicitly including upstream in the name to differentiate between this and the client<->server keepalive settings, though this could simply be ingress.kubernetes.io/keepalive).

I might wind up trying to implement this myself, but I figured I should go ahead and open the feature request regardless. I searched open issues and don't think I saw anything like this; apologies if this is a duplicate feature request.

[rikatz]

@vdavidoff Let me know if you need any help implementing this :)

[aledbf]

@vdavidoff let's start adding a global parameter in the configuration. We can add an annotation after we verify this works without issues.

[vdavidoff]

Thanks!