When JSON model's no_paramfile attribute is given, check for its value

moved discussion from https://github.com/boto/botocore/pull/245

Before this fix, only existence of no_paramfile attribute was checked and
its value(true/false) was not checked.
So, as long as no_paramfile attribute was defined in JSON model,
"no_paramfile": true and "no_paramfile": false behaved the same way.