Map REMOTE_AUTH_BACKEND env var to list

[NeodymiumFerBore]

Related Issue: None

New Behavior

REMOTE_AUTH_BACKEND is now mapped as a list in configuration.py, so we can configure multiple remote auth backends without using extra.py.

Example use case: use LDAP and social backends.

Contrast to Current Behavior

As of today, REMOTE_AUTH_BACKEND environment variable is not mapped to any specific type, just a string. To configure multiple backends, we have to set our REMOTE_AUTH_BACKEND list in extra.py.

Discussion: Benefits and Drawbacks

Benefits

• Can configure multiple remote auth backends via environment variable, without any addition in extra.py
• Backward compatibility: it will not break existing configurations:
  • Single backend set as env var: will load a single element list
  • Set as extra conf: the extra conf will still supersede the new definition
• the upstream settings.py casts REMOTE_AUTH_BACKEND to a list if it is not a list or tuple, before copying *REMOTE_AUTH_BACKEND into AUTHENTICATION_BACKENDS, so nothing has to be done in netbox-docker to carry this backward compatibility
• Parameters for REMOTE_AUTH_BACKEND being python import paths, they will not contain white spaces. The _AS_LIST lambda should work as expected in all cases

Drawbacks
I don't see any

Changes to the Wiki

Upstream netbox documentation about REMOTE_AUTH_BACKEND should specify that configuring multiple remote auth backends with a list is possible (it's already the case).

I don't see any required change in the netbox-docker wiki.

Proposed Release Note Entry

REMOTE_AUTH_BACKEND environment variable can now define multiple backends, separated with white spaces.

Double Check

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• My PR targets the develop branch.