Config boolean environment variables need to be cast to ints, since
they are set or unset using 0 and 1. Previously they were interpreted as
(pseudocode) read_var(name, default=False), which meant that setting
CONFIG_VAR=0 would enable that variable since Python reads environment
variables as strings, and '0' is truthy. This updates the previous logic
to (still pseudocode) int(read_var(name, default='0')).
Fixes#279
baa7a87efb