[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
PR Review

- going to semver versioning, so 1.1.0-pre

https://trello.com/c/PkUsXZx1
Merge pull request #1134 from abellotti/rest_api_vm_accounts

[api] Adding support for VM Accounts subcollection