accounts role

This role:
- creates groups from the `accounts_groups` list
- creates/removes users from the `accounts_users` list
- optionally creates any users declared in `accounts_groups[].members` when `create_missing_users: true`

Vars (examples):
- `accounts_groups` (list)
- `accounts_users` (list)
- `create_missing_users` (bool, default: false)

Usage: include the `accounts` role in a play or run `playbooks/accounts.yml`.