fix unmarshaling of expanded environment variables with special characters (#3770)

If we expand environment values directly with os.ExpandEnv() over whole
config, we might end up in a situation where the environment variable
has escape characters that break the resulting JSON, and unmarshalling
fails. Instead of expanding the entire config with single call, we
recurse through the config and expand the values in leaves one by one.

Signed-off-by: Tuomo Tanskanen <tuomo.tanskanen@est.tech>