Use yaml.safe_load and yaml.safe_dump

[xvello]

When loading a kubeconfig file, the library uses the unsafe yaml.load method, which is vulnerable to arbitrary code execution, as described in https://nvd.nist.gov/vuln/detail/CVE-2017-18342

This PR makes sure we use the safe_load method. For consistency, I also change test to use the safe_dump method, although there is probably no risk there.

The use case for the unsafe load is to load data in arbitrary python classes, but in this case we unmarshal the file in a classic dict. safe_load has what we need for this use case.

[codecov-io]

Codecov Report

Merging #111 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #111   +/-   ##
=======================================
  Coverage   92.04%   92.04%           
=======================================
  Files          13       13           
  Lines        1182     1182           
=======================================
  Hits         1088     1088           
  Misses         94       94

Impacted Files	Coverage Δ
config/kube_config.py	84.24% <ø> (ø)	⬆️
config/kube_config_test.py	94.41% <100%> (ø)	⬆️

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[micw523]

/lgtm

[yliaog]

/lgtm

[yliaog]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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