Automatic merge from submit-queue
Allow customisation of the nginx proxy_buffer_size directive via ConfigMap
I'm opening a new PR with the same changes as #1693 because I pushed the latter with an email address that can't be used to sign the CLA. Description from the previous PR:
When using nginx as a proxy we can run into the following error:
```
upstream sent too big header while reading response header from upstream
```
In order to fix this, we need to be able to configure the proxy_buffer_size nginx directive to increase its value. This PR updates the nginx-ingress-controller to allow that.
77afc22875