Don't cache the node config in the CfgVars struct

[twz123]

Description

The node config should be read at most once and then reused. Caching the config in this way shouldn't be necessary.

On the way:

• Use only the path as parameter to LoadRuntimeConfig. This is the only information needed. Passing just the path instead
  of the full CfgVars struct makes it easier to reason about what is happening.
• Remove unused vars variable and DeepCopy. The vars variable was never read, and the DeepCopy method was used just to initialize it. Remove both. The original intent was probably not to list the StartupConfigPath in the file written to disk, but since that never worked as intended and there were no problems with it, I think it's okay to leave things as they are.
• Restructure NewRuntimeConfig test. Use a real config file instead of mocking out the config file loading
  using the private nodeConfig field. Verify that the folder for the runtime config file is created, if it doesn't exist.

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

[ncopa]

LGTM