Enable keep-alive requests to upstreams

[f84anton]

What this PR does / why we need it:
Set upstream proxy header "Connection: keep-alive" if upstream-keepalive-connections greater then 0 and we do not have Upgrade header in request.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #986

Special notes for your reviewer:

Release note:

NONE

[k8s-reviewable]

This change is 

[coveralls]

Coverage remained the same at 37.141% when pulling 9a21482 on f84anton:patch-1 into dcf4a45 on kubernetes:master.

[aledbf]

@f84anton please add an e2e test for this scenario

[f84anton]

@aledbf, what kind of test? To make sure that connection to upstream send with Connection: keep-alive? I'm not familiar with e2e tests and I have weak golang skill. Need more hints.

[aledbf]

@f84anton please check https://github.com/kubernetes/ingress-nginx/blob/master/test/e2e/defaultbackend/default_backend.go
and https://github.com/kubernetes/ingress-nginx/blob/master/test/e2e/annotations/alias.go
Basically you need to check the new code is generated in the nginx.conf and make some requests with aseertions (like searching in the headers the keepalived value)

[aledbf]

I think we need a "connection" annotation to just allow to override the Connection header