feat: merging kubeconfig files

[tomplus]

I'm trying to add support for multiple kubeconfig file. I prepared an additional class which merges configs and produces ConfigNode which can be passed directly to KubeConfigLoader. KubeConfigLoader and other public methods are backward compatible. I had to extend the ConfigNode by adding path to source kubeconfig too, because paths to certificates are relative.

Please let me know what you think.

Related issue: #62

[codecov-io]

Codecov Report

Merging #94 into master will increase coverage by 0.59%.
The diff coverage is 97.05%.

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
+ Coverage   92.04%   92.64%   +0.59%     
==========================================
  Files          13       13              
  Lines        1182     1264      +82     
==========================================
+ Hits         1088     1171      +83     
+ Misses         94       93       -1

Impacted Files	Coverage Δ
config/kube_config_test.py	94.82% <100%> (+0.4%)	⬆️
config/kube_config.py	86.74% <95.52%> (+2.49%)	⬆️

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[tomplus]

@roycaihw could you take a look?

[tomplus]

@roycaihw Have you had time to look into this?

[roycaihw]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: roycaihw, tomplus

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[tomplus]

@roycaihw Thank you!