we should not ask aws credentials, everytime we run `magemaker --cloud aws`.

[eff-kay]

Describe the bug
we should not ask aws credentials, everytime we run magemaker --cloud aws.

To Reproduce
Steps to reproduce the behavior:

1. run magemaker --cloud aws, and follow the steps
2. run it again, it asks for the same credentials.

Expected behavior
we should pick the credentials from .env file. If the user wants to update them we can rewrite them.