DefaultConfigurationMixin: Modify implementation

[nvzard]

Current implementation is either DEFAULT_CONFIG or CONFIG_TEMPLATE. But a better implementation will be if we can store the default values in CONFIG_TEMPLATE and then allow a subset of those config variables overridden in the config.py's DEFAULT_CONFIG.

This way we can provide the partial configuration in the CONFIG_TEMPLATE, and allow partial reconfiguration through config.py's DEFAULT_CONFIG.