Refactor creation of the ~/.aws/shell/ directory

> We might want to consolidate the places where we create the ~/.aws/shell/ directory (and sub directories). I count four separate locations now in the codebase (including this new call) where we are calling os.makedirs.

As initially discussed in https://github.com/awslabs/aws-shell/pull/62.

When we do consolidate it seems cleaner to go with the try/catch approach vs the check/create approach. Here's a good [StackOverflow post](http://stackoverflow.com/a/5032238) with details.