Allow for IAM role creation to be bypassed and use pre-created roles

Summary

Some enterprise customers that will be implementing GitLab do not allow the creation of IAM roles directly and/or the process is controlled by another team. This significantly inhibits the customers ability to leverage the awesome automation and continuous updates that make GET so useful.

These customers are having to maintain their own customized version of GET to meet their requirements which makes keeping it up to date very difficult. One of the most frequent customizations is the create of IAM roles and policies.

Proposed Solution

For Linux Package (Omnibus) environments GET configures IAM resources for VMs by default only when Object Storage is concerned. IAM Profiles and Roles are not actually required though otherwise so a reachable solution here is to add an option to configure GET to create the Object Storage but not create the IAM resources for authentication.

This option will allow customers to configure authentication separately as desired, including alternative auth methods.