Fix buildupstream name to work with dynamic session affinity

[zrdaley]

What this PR does / why we need it:

When dynamic configuration is enabled, ngx.var.proxy_upstream_name is used to get the current backend (stored on the NGINX configuration) in the balancer lua module. This variable is created in buildUpstreamName() in /internal/ingress/controller/controller.go.

PROBLEM: Backends stored in the NGINX configuration do not use the convention sticky-<upstream-name> when dynamic configuration is enabled. If ngx.var.proxy_upstream_name uses the sticky convention this will cause a 500 internal server error.

This PR prevent buildUpstreamName from using the naming convention sticky-<upstream-name> when dynamic configuration is enabled prevent 500 internal server errors.

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

Special notes for your reviewer:
This is a prerequisite for dynamically configured sessions affinity [INSERT PR HERE]

[aledbf]

/approve

[ElvinEfendi]

you need to update buildUpstreamName call at https://github.com/Shopify/ingress/blob/9198e2c14b616414d047ab733d4a2f29a464a02b/rootfs/etc/nginx/template/nginx.tmpl#L775 too

[codecov-io]

Codecov Report

Merging #2340 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2340      +/-   ##
==========================================
+ Coverage   37.81%   37.86%   +0.05%     
==========================================
  Files          72       72              
  Lines        5144     5145       +1     
==========================================
+ Hits         1945     1948       +3     
+ Misses       2902     2901       -1     
+ Partials      297      296       -1

Impacted Files	Coverage Δ
internal/file/bindata.go	55.78% <ø> (ø)	⬆️
internal/ingress/controller/template/template.go	65.45% <100%> (+0.5%)	⬆️

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[aledbf]

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, zrdaley

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[aledbf]

@zrdaley thanks!