Add option to configure cooldown time

[christopher-drews]

In order to avoid repeated restarts during automated tests, please add a possibility to configure the cooldown time. As of now this is only relevant for test scenarios therefore a global setting could be sufficient.

Suggestion: in scaler/mod.rs:

//const COOLDOWN_PERIOD_SECONDS: u64 = 60;
const COOLDOWN_PERIOD_SECONDS: u64 = env_var!(optional "COOLDOWN_PERIOD_SECONDS",default: "60").parse::<u64>().expect("Parsing env var COOLDOWN_PERIOD_SECONDS");