Fix json-cfg syntax

[RyanJarv]

This fixes the syntax for json-cfg when running --aws-managed-policies and --auth-details-file

Before this commit you would get an error similar to the following:

    jsoncfg.config_classes.JSONConfigNodeTypeError: Expected a ConfigJSONObject but found
    ConfigJSONScalar. You are trying to access the __contains__ magic method of a scalar
    config object.

It seems that instead of treating the json-cfg objects as hash's you're supposed to access keys
via attributes and use a method call when you want to convert it back into it's original form.

Passing an argument to this method call sets the default return, acting similar to:

    obj.get('key', default)

[CLAassistant]

All committers have signed the CLA.

[0xdabbad00]

LGTM, thank you @RyanJarv.

This is ready to merge @steiza