iam_role and role_arn are confusing names that are easily confused.

[jfalkenstein]

Subject of the issue

Right now, Sceptre has two very confusingly named StackConfigs that are easy to mistake for each other, despite operating very differently:

• role_arn is the ARN of the CloudFormation Service Role attached to a stack (permanently) and is assumed by CloudFormation to perform all stack actions.
• iam_role is the ARN of the role Sceptre assumes and uses to perform all stack actions.

**I believe we should deprecate these names and issue deprecation warnings when these configurations are used. Instead, we should transition to using much clearer names. I propose:

• cloudformation_service_role
• sceptre_deployment_role

Ideally, we'd deprecate the old configs (probably along with template_path), issue a warning, and then assign those values to the new fields. We could remove the deprecated fields on the next major version.

[zaro0508]

I am in favor of this idea.