Explicitly cast config types on fakebackend.

Better mimics what happens in the real backend. Also fixes code that expects a 'false' config value to be evaluated as false, which is what was causing the original bug:

https://launchpad.net/bugs/1371789
I think it'd be best to have a value for each data type: type can be string, int, float, or boolean and when the code changes the value to have both the positive and negative case in it. 

https://juju.ubuntu.com/docs/authors-charm-config.html