Support for custom log-format

[df-mreidel]

I noticed that the log-format could not be customized and created a small patch that makes exactly this possible so everyone can add fields to his or her liking or switch to a totally different format. Use like this to switch to JSON-logging for example:

apiVersion: v1
kind: ConfigMap
metadata:
  name: nginx-ingress-conf
  namespace: kube-system
data:
  log-format: '{ "@timestamp": "$time_iso8601", "@version": "1", "clientip": "$remote_addr", "tag": "ingress", "remote_user": "$remote_user", "bytes": $bytes_sent, "duration": $request_time, "status": $status, "request": "$request_uri", "urlpath": "$uri", "urlquery": "$args", "method": "$request_method", "referer": "$http_referer", "useragent": "$http_user_agent", "software": "nginx", "version": "$nginx_version", "host": "$host", "upstream": "$upstream_addr", "upstream-status": "$upstream_status" }'

I also noticed that the tempalte engine was html/template, which caused all text inside single quotes to be escaped like it was an HTML attribute. I therefore switched to text/template, I think this was meant to be used anyway.

[pleshakov]

@df-mreidel
Thx! That's very useful.
html/template was used by mistake. Thanks for fixing it.