Create AWS_ASSUME_ROLE_ARN variable

Hi,

It would be awesome if when we use AssumeRole system we could get the ARN in a variable.

My need is that for run terraform or some other tools which are able to assume role by them self I would not need to duplicate the ARN in jenkins and in the tools...

What do you think ?

Cheers
Geoffrey

No files or tests are named. Start by locating the plugin's AssumeRole handling and how credential values are exposed to builds; define done as making the assumed role ARN available through AWS_ASSUME_ROLE_ARN so tools such as Terraform can consume it without duplicating the ARN, with coverage for the new variable.

Written by the indexing model from the issue text.