fix(agent): use openebs' lib-csi k8s client to load kubeconfig

[dionysius]

Pull Request template

Why is this PR required? What issue does it fix?: The binary crashes if you want to use it with a kubeconfig stating it can't find the kubeconfig. The package variables kubeconfig and masterurl were never holding a value.

What this PR does?: Uses existing implementation to find kubeconfig. This is already in use in package zfsnode.

Does this PR require any upgrade changes?: no

If the changes in this PR are manually verified, list down the scenarios covered:: starts up binary successfully with env OPENEBS_IO_KUBE_CONFIG set and in agent plugin mode.

Any additional information for your reviewer? : no (see linked issue)

Checklist:

• Fixes node agent: allow running out of cluster using $KUBECONFIG #414
• PR Title follows the convention of <type>(<scope>): <subject>
• Has the change log section been updated?
• Commit has unit tests
• Commit has integration tests
• (Optional) Are upgrade changes included in this PR? If not, mention the issue/PR to track:
• (Optional) If documentation changes are required, which issue on https://github.com/openebs/website is used to track them:

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.91%. Comparing base (8c402d3) to head (d4ba8de).
Report is 55 commits behind head on develop.

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #645      +/-   ##
===========================================
- Coverage    95.99%   95.91%   -0.08%     
===========================================
  Files            1        1              
  Lines          574      686     +112     
===========================================
+ Hits           551      658     +107     
- Misses          19       23       +4     
- Partials         4        5       +1     

Flag	Coverage Δ
bddtests	95.91% <ø> (∅)

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[Abhinandan-Purkait]

LGTM

[Abhinandan-Purkait]

@dionysius Are we good to merge?

[Abhinandan-Purkait]

Will merge once CI passes. Thank you!