Source credentials from the environment

```
$ aws-vault exec my-account
$ aws sts get-caller-identity # works
$ awspx ingest
[16/02/23 15:36:14] NOTICE   The profile 'default' doesn't exist. Please enter your AWS credentials.
                             (this information will be saved automatically)
AWS Access Key ID [None]:
AWS Secret Access Key [None]:
Default region name [None]:
Default output format [None]:
```