Let weep use existing credentials

[patricksanders]

Certain current and future weep commands could be useful apart from retrieving credentials from ConsoleMe. For example:

• the ecs_credential_provider could accept a profile name and serve credentials from the AWS Shared Credentials file, e.g. AWS_CONTAINER_CREDENTIALS_FULL_URI=http://localhost:9091/ecs/profile/test_profile
• the assume role functionality could be used with credentials in environment variables/shared credentials file, e.g. weep export --from-env -a arn:aws:iam::123456789012:role/other_role or weep export --from-profile test_profile -a arn:aws:iam::123456789012:role/other_role

The UX will need some ironing out to make it more intuitive, but hopefully this captures the spirit well enough.