sysctl: parse bool value as 0/1 and refactor a little bit

Kernel parameters uses `0` and `1` to represent a boolean values, so it’s convenient to parse “Ansible boolean” (true, yes, on, …) or YAML boolean converted to string (True, False) as 0, or 1. It simplifies writing of parametrized roles when sysctl parameters are user modifiable variables; you don’t need an explicit conversion in a task.