AWS: Ability to assume a role in secondary account was not included in aws-sd

[jdeprin]

The ability to assume an IAM role in a seconday account was added in: #524. This however only added it to the aws provider but left it out of the aws-sd provider.

Can this functionality be moved over so that both R53 and service discovery can be accessed through an assumed role?

https://github.com/kubernetes-incubator/external-dns/blob/master/provider/aws.go#L111
https://github.com/kubernetes-incubator/external-dns/blob/master/provider/aws_sd.go#L72

[njuettner]

@vanekjar What do you think? Can this be addressed?

[goatsthatcode]

I just submitted a PR to address this, I have signed the CLA agreement but it does not seem to have registered yet in their system. Please let me know if there is anything else I can do to expedite this.