Sample rate configmap option for zipkin in nginx-opentracing

[gugu]

What this PR does / why we need it:

Currently it is not possible to specify sample rate for zipkin in opentracing plugin, which would be really usable as ingress is a first entrypoint in whole microservice chain

This pull request adds an option zipkin-sample-rate in ConfigMap

[codecov-io]

Codecov Report

Merging #2719 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2719      +/-   ##
==========================================
+ Coverage   40.95%   40.96%   +0.01%     
==========================================
  Files          72       72              
  Lines        5084     5085       +1     
==========================================
+ Hits         2082     2083       +1     
  Misses       2713     2713              
  Partials      289      289

Impacted Files	Coverage Δ
internal/ingress/controller/nginx.go	10.93% <ø> (ø)	⬆️
internal/ingress/controller/config/config.go	98.24% <100%> (+0.01%)	⬆️

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[gugu]

/assign @bowei

[aledbf]

@gugu please don't assign approvers manually

[aledbf]

/approve
/lgtm

[aledbf]

@gugu thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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