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.
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
This strips out http.DefaultClient everywhere I could immediately find
it. Too many things use it and then modify it in incompatible ways.
Fixes#700, I believe.
