[terraform] Rename aws_ecs to keyserver_cluster and move shared security group

Summary:
Planning on making aws_ecs the shared file for config options shared between keyserver_primary and keyserver_secondary.

Depends on D12863

Test Plan: terraform apply. No changes planned

Reviewers: bartek, varun

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12868