Allow setting daemons via environment variables about openpilot

[mgraczyk]

Instead of this many environment variables, it would probably be more appropriate to use a config file. We pass around a globally shared immutable context called gctx throughout the code. We could include these values and load gctx from a file in manager_init. @geohot what do you think?

[ingenieroariel]

Has this been addressed in later internal releases?

I have my own boardd and would love to just pass the right configuration to swap it.

[geohot]

Internally we just edit manager. We have a few internal services now too, maybe we'll have an external file with the service list in 0.3

[espes]

This can now be done with manager_extensions.py