replace str2bool with validate_bool

When working on the sensu puppet module one of the module users pointed out that the latest PE only supports stdlib 3.2.0 and str2bool was introduced in stdlib 4.0.0.  I'm not aware of any users this is actively presenting a problem for, but using str2bool does prevent PE users from using this module.  #77 highlighted this was in use for me.

Feel free to just close if PE support isn't needed.