Add elb-certificate-arn()

[jonhiggs]

No description provided.

[mbailey]

Curious why this function includes awscli call instead of calling elbs()
This pattern is used throughout instance-functions

I actually don't mind repetition as it makes the function self contained. I would ideally want the project to use one or the other though.

[jonhiggs]

You're right. Let me fix it.

The reason to fold it back was because I planned to implement an output parse. Probably column for text and something else for json.