Allow login with master creds in environment

[mtibben]

Use a federation token when master creds are sourced from the environment

Fixes #886

[christophetd]

I tested this branch while being authenticated as an IAM role (after a successful aws-vault exec), and it doesn't seem to be working as I would expect:

$ aws-vault exec foo
$ aws sts get-caller-identity
{
    "UserId": "XXX:christophetd",
    "Account": "012345678912",
    "Arn": "arn:aws:sts::012345678912:assumed-role/admin/christophetd"
}
$ git checkout login-with-master-creds
$ go run . login
aws-vault: error: login: operation error IAM: GetUser, https response error StatusCode: 400, RequestID: 78c10110-6778-44cc-ac8a-e41ca01b3613, api error ValidationError: Must specify userName when calling with non-User credentials

[mtibben]

I tested this branch while being authenticated as an IAM role (after a successful aws-vault exec), and it doesn't seem to be working as I would expect:

@christophetd OK I think I fixed that now

[christophetd]

Works well with env variables now! (it doesn't with --server or --ecs-server FWIW). Great stuff!