Make it easier to override config via env vars

Although technically enabled, environment vars are rather useless at the
moment. Allow to override config values via specifying a distinct
prefix (to distinguish from underscore in the variable names), and
enable parsing for working types other than string.

src/main.rs

@@ -33,8 +33,8 @@ fn main() {
         .add_source(config::File::with_name("/etc/berserker/workload.toml").required(false))
         .add_source(config::File::with_name("workload.toml").required(false))
         // Add in settings from the environment (with a prefix of APP)
-        // Eg.. `WORKLOAD_DEBUG=1 ./target/app` would set the `debug` key
-        .add_source(config::Environment::with_prefix("WORKLOAD"))
+        // Eg.. `BERSERKER__WORKLOAD__ARRIVAL_RATE=1` would set the `arrival_rate` key
+        .add_source(config::Environment::with_prefix("BERSERKER").try_parsing(true).separator("__"))
         .build()
         .unwrap()
         .try_deserialize::<WorkloadConfig>()