Draft: Refactor nomad config management by using pydantic models

Closes #1353

• using pydantic model BaseSettings
• make sure that the dictionaries will be merged and all other type will be overwritten
• make sure that config is a singleton object
• make sure that we are loading the settings in the right order :
  • values from __init__
  • env vars
  • nomad.yaml (from NOMAD_CONFIG OR from cwd (OR from the project folder)?)
  • all the default values
• update docs about the usage of double underscore for env vars (NOMAD__MONGO__HOST)
• update tests