Support use cases where you want to provision STS tokens
using Vault, but, you need to call AWS APIs that are blocked
for federated tokens. For example, STS federated tokens cannot
invoke IAM APIs, such as Terraform scripts containing
`aws_iam_*` resources.
1) Use the new LeaseExtend
2) Use default values controlled by mount tuning/system defaults instead
of a random hard coded value
3) Remove grace periods
The secretAccessKeysRevoke revoke function now asserts that it is
not dealing with STS keys by checking a new internal data flag. Defaults
to IAM when the flag is not found.
Factored out genUsername into its own function to share between STS and
IAM secret creation functions.
Fixed bad call to "WriteOperation" instead of "UpdateOperation" in
aws/backend_test
The new STS path allows for obtaining the same credentials that you would get
from the AWS "creds" path, except it will also provide a security token, and
will not have an annoyingly long propagation time before returning to the user.
