Default registry config updated.

[janosmurai]

Type Of Change

• New feature (non-breaking change which adds functionality)
• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Any modification in the test cases
• Any modification in the documentation

Checklist:

• I have read and followed the contribution guideline.
• I have checked to ensure there aren't other open Pull Requests for the same update/change.
• All the test cases pass and new modifications in the production code are 100% covered.
• I have made corresponding changes to the documentation.

Related Issue

Closing:
DEM-292

Description

Configs for Docker Hub registries must include a name, namespace and url
fields since v0.7.0, but the default config wasn't updated accordingly.

How Has This Been Tested?

Tested on Ubuntu 22.04.
I have removed the ~/.config/axem directory and run the dem list command. DEM
recreated the removed directory with the correct config file.