Refactor kubelet config generation

[twz123]

Description

Rename prepareLocalKubeletConfig to writeKubeletConfig and have this method not only generate the configuration data, but also write it to the designated file. Makes the Start method a bit shorter, and keeps related code snippets closer together. In this particular case, this eliminates an unnecessary string conversion back and forth. Move all kubelet config logic into that method, as well. This allows for dissolving the kubeletConfig struct and make the assignments directly to the generated KubeletConfiguration struct. Removes unnecessary indirection and concentrates more of the decision making into the appropriate method. Defer OS-specific decisions about the path to resolv.conf to the specific function, so that its return value is meaningful even if it's not called on Linux. Set the cgroups settings and do systemd detection only on Linux, as these are Linux-specific and don't exist on other operating systems.

Type of change

• 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 not work as expected)
• Documentation update

How Has This Been Tested?

• Manual test
• Auto test added

Checklist:

• My code follows the style guidelines of this project
• My commit messages are signed-off
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have checked my code and corrected any misspellings