* iam_cert.py Fix duplicate certificate detection with included chains.
The iam_cert module would fail to detect certificates as duplicates
if the certificate body included the authority chain directly.
This commit fixes the problem by checking if a given certificate
matches the start of the data returned by AWS, since in all cases
where they would match the certificate will come first.
* iam_cert.py Return certificate ARN in all success cases.
When uploading certificates or interacting with IAM, the certificate ARN
is needed for other operations with AWS such as provisioning elastic load
balancers.
This commit returns the certificate ARN in all success cases, which allows
it to be used to idempotently provision other Amazon services depending on
it (ELBs being an immediate example).
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