Add support for cross account deployment

[mvanbaak]

Right now you can either use the instance profile, or an access/secret key combination.
Our usecase is: jenkins runs in AWS account A and needs to deploy lambda functions in account B.

The codedeploy plugin of jenkins allows for specifying an iamRole to assume in the aws sdk.
Would be great if this plugin supports this as well.

Example of the codedeploy plugin:
iamRoleArn: 'arn:aws:iam::XXXX:role/some-role'