MINOR: lua: change tune.lua.log.stderr default from 'on' to 'auto'

MINOR: lua: change tune.lua.log.stderr default from 'on' to 'auto'

After making it configurable in previous commit "MINOR: lua: Add flags
to configure logging behaviour", this patch changes the default value
of tune.lua.log.stderr from 'on' (unconditionally forward LUA logs to
stderr) to 'auto' (only forward LUA logs to stderr if logging via a
standard logger is disabled, or none is configured for the current context)

Since this is a change in behaviour, it shouldn't be backported