WIP: Dynamic configuration fixes

[aledbf]

• add additional check here to avoid passing probes before initial sync
• check is not required to re-send configuration after a reload (ingress backend successfully reloaded) force backend sync when worker starts #2511
• cleanup JSON structure before making request to lua
• check required client_body_buffer_size to avoid temporal files

[mikksoone]

External authentication doesn't seem to work:

"POST /path HTTP/1.1" 200 0 "-" "Main (Main)" 0 0.975 [external-authentication] 127.0.0.1:8886 0 0.975 200 a158d8a6e08c556a1fad5c39aeed28f6
2018/03/21 21:06:12 [error] 178#178: *728 failed to run balancer_by_lua*: /etc/nginx/lua/balancer.lua:31: attempt to index local 'backend' (a nil value)
stack traceback:
	/etc/nginx/lua/balancer.lua:31: in function 'balance'
	/etc/nginx/lua/balancer.lua:97: in function 'call'
	balancer_by_lua:2: in function <balancer_by_lua:1> while connecting to upstream, client: IP, server: server, request: "POST /path HTTP/1.1", host: "host"

[ElvinEfendi]

@mikksoone please create a separate issue with the logs based on instructions at #2225 (comment) and #2225 (comment)

[ElvinEfendi]

check required client_body_buffer_size to avoid temporal files

@aledbf I see this is checked now. Where was it implemented? We are currently working on making it to work with the edge case when Nginx can not fit the body into buffer and stores it in temporary file.

UPDATE: I see https://github.com/kubernetes/ingress-nginx/pull/2309/files now.