config: Deprecated SWHConfig in favor of load_from_envvar function

That new function declares exactly what happens today during our configuration
loading (for loader/lister/indexer/... and rpc services in general not for the
CLIs which will be dealt with later).

Loads and parses the yaml configuration file out of the SWH_CONFIG_FILENAME
environment variable. Allowing eventual dict enrichment with default
configuration if provided.

Related to T1532