Add label selector for plugin

[aca]

What this PR does / why we need it:

fix #4950
Nginx-ingress-controller is often deployed as daemonset. Current implementation only supports pod / deployment options. It will be much flexible if we can use label selector for controller.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Which issue/s this PR fixes

#4950

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added tests to cover my changes.
• All new and existing tests passed.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @aca. Thanks for your PR.

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[aledbf]

/ok-to-test

[aledbf]

/lgtm
/approve

[aledbf]

@aca thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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

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[aledbf]

/retest

[aledbf]

/retest

[aledbf]

/retest

[codecov-io]

Codecov Report

Merging #5080 into master will increase coverage by 0.69%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master   #5080      +/-   ##
=========================================
+ Coverage    58.5%   59.2%   +0.69%     
=========================================
  Files          88      88              
  Lines        6866    7143     +277     
=========================================
+ Hits         4017    4229     +212     
- Misses       2409    2463      +54     
- Partials      440     451      +11

Impacted Files	Coverage Δ
internal/ingress/metric/collectors/process.go	88.54% <0%> (-2.09%)	⬇️
internal/ingress/controller/endpoints.go	93.33% <0%> (-0.42%)	⬇️
internal/ingress/controller/controller.go	53.04% <0%> (+3.1%)	⬆️
internal/ingress/controller/store/store.go	62.6% <0%> (+5.58%)	⬆️

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