[FEATURE] Add Environment Variables to portal_config

[msouff]

Describe the solution you'd like

portal_config.yml

settings:
  ...
  ENVIRONMENT_VARIABLES:
    VAR1: "val1"
    VAR2: "val2"
  ...

settings.py

...
# Add environment variables
ENVIRONMENT_VARIABLES = portal_config_settings.pop("ENVIRONMENT_VARIABLES", {})
for var, val in ENVIRONMENT_VARIABLES.items():
    os.environ[var] = val
...

Additional context
This would make it easier to add or overwrite portal and app related environment variables and keep track of them in the portal_config.