[api] Adding support for VM Accounts subcollection

- Access via /api/vms/#/accounts subcollection
- Also via /api/vms/# --expand=accounts
- Added spec/requests/vms_spec.rb to test accounts subcollection

I need to bucket all v2.0 work before v2.0 gets released and I didn't want this to be v1.0, so PRs will not each get a new version, I just picked 1.1.