add missing REMOTE_AUTH config options

[hendrikbl]

Related Issue: #992

New Behavior

All REMOTE_AUTH condifuration settings are now configurable via environment variables.
A list of all available options can be found here: https://github.com/netbox-community/netbox/blob/develop/docs/configuration/remote-authentication.md

Contrast to Current Behavior

Currently only the following optiosn are configurable via environment variables:

REMOTE_AUTH_ENABLED
REMOTE_AUTH_BACKEND
REMOTE_AUTH_HEADER
REMOTE_AUTH_AUTO_CREATE_USER
REMOTE_AUTH_DEFAULT_GROUPS

Discussion: Benefits and Drawbacks

This change makes configuration more consistent since most of the configuration should be done in .env files if possible.

Changes to the Wiki

None

Proposed Release Note Entry

Make all REMOTE_AUTH config options configurable via environment variables

Double Check

• I have read the comments and followed the PR template.
• I have explained my PR according to the information in the comments.
• My PR targets the develop branch.

[tobiasge]

Please use the provided PR template an target the develop branch.

[hendrikbl]

Please use the provided PR template an target the develop branch.

Oh sorry, don't know how I messed this up. I updated the PR with the correct template.

[empusas]

Why not also include the most common SSO integrations variables as suggested in this issue:
#1139

[hendrikbl]

Why not also include the most common SSO integrations variables as suggested in this issue: #1139

I'm not sure if it's a good idea to knowingly put only a few configuration parameters for SOCIAL_AUTH_ in there. I think it leads to confusion if some of them are configured via env and some are not.
But that's something the maintainers have to decide I guess @tobiasge

[cimnine]

Thanks for the PR. This lgtm.