Allow for storing secret_key values in system keychain

Boto supports the ability to store your secret_key using the system keychain/keyring service.

For example, your ~/.boto file may contain the following

```
[Credentials]
aws_access_key_id=
keyring=aws
```

This makes it possible to keep at least your secret key encrypted.

I just started going through the code for this, and it looks like that isn't an option with the aws-cli configuration commands at this point.