Implement passing configuration over environment variables

[kpaleniu]

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PR Summary

This pull request adds functionality for overriding configuration file content from environment variables.
For example:

$ SLACK_TOKEN="xoxb-foo-bar" REPEAT_PLUGIN_DUMMY_VARIABLE="true" rtmbot

This will result in effectively the following rtmbot.conf:

SLACK_TOKEN: "xoxb-foo-bar"
plugins.repeat.RepeatPlugin:
    dummy_variable: "true"

All rtmbot-specific configuration variables can overridden. For plugin-specific configuration
I left the limitation of treating all values as strings to keep code from getting overly complex.

All this is mentioned in the README.md that is also updated by this pull request.

Related Issues

Implements at least the environment variable portion of #89.

Test strategy

tests/test_rtmbot_runner.py includes unit tests that cover most usual cases that came to mind.

[CLAassistant]

All committers have signed the CLA.

[nyukhalov]

Hi. What is the ETA for merging the change?
I really miss this feature #86

[fzipi]

@Roach I think this should be merged. Having config in env vars is critical when running rtmbot in containers and/or kubernetes.

[iangoldsmithsf]

@Roach could you merge this please - from the Perceptyx version I saw it also seemed to incorporate allowing the log to be set to STDOUT - which is also crucial.

Containers don't like log-files - the environments are ephemeral, but typically have good logging of stdout etc.