remove unnecessary variables and use roles instead of keys for AWS Encryption-AtRest

[nikhil-mongo]

remove unnecessary variables and use roles instead of keys for AWS Encryption-AtRest

Description

This improves the Encryption at Rest documentation and example cleanup. Fixes #424

Link to any related issue(s):

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the Terraform contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

[nikhil-mongo]

Update the link

[themantissa]

LGTM but because the docs now link to the examples it would be great if you can create one for v0.9.0 (we are close to releasing) because it has a new way to do cloud provider access with one apply. Thanks!!