Use a map for root_block_device instead of list

[Yethal]

Is your request related to a new offering from AWS?

Is this functionality available in the AWS provider for Terraform? See CHANGELOG.md, too.

• No 🛑: please wait to file a request until the functionality is avaialble in the AWS provider
• Yes ✅: please list the AWS provider version which introduced this functionality

Is your request related to a problem? Please describe.

Instead of using a list of maps for root_block_device I'd like a map to be used. There can't be more than one root_block_device anyway and turning it into a single map will make it easier for terraform wrappers to merge in configuration
As suggested in #396, I'm opening an issue instead of keeping PR open.

Describe the solution you'd like.

Describe alternatives you've considered.

Additional context