Remove storage parameter from DefaultClusterConfig

[twz123]

Description

The storage override occurs only during the initial loading of the node configuration, and can be modeled by applying the changes to the default configuration and then merging the user-provided configuration data into it.

Also remove ConfigFromReader: The function hid the fact that there was no benefit at all to providing the input as a reader. It merely used io.ReadAll to load the data into memory. Removing this, and exposing to callers the fact that the only supported input is just a byte slice, makes the code more straightforward.

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