Specify path to token in env var

[MrCreosote]

Currently there are 2 methods to get a token into the callback server:

https://github.com/kbase/JobRunner/blob/main/JobRunner/config.py#L12-L22

• an env var (e.g. -e KB_AUTH_TOKEN=token), which might show up in ps output
• a file called token in the cwd.

Add an env var that specifies the path to a token file.