support environment variable expansion in log config

[wasim-nihal]

we have a use case where we spin up 3 different kiwigrid k8s-sidecars. And each of this sidecar has its own log config mounted as a config map. It would be better if there is a feature in kiwigrid-k8s sidecar to expand environment variables and populate the log config at the startup. This would enable users to have only only log config for multiple k8s sidecars and the values can be configurable via environment variables.

Example:
The expansion can happen based on a specific placeholder in the log config (To instance $(<some_env_variable>).

The user can configure the log config in the below way:

version: 1
disable_existing_loggers: false

root:
    level: $(LOG_LEVEL)
    handlers: [console]

And in different sidecars, the value of the environment variable can be different according to needs.

Note: This feature will not be limited to LOG_LEVEL. It shall be generic enough to look up any environment variable enclosed within the placeholder $().