Add user "etcd" to role "root" so that the default tls client works

[hvgirish]

The default tls client that is created with CN "etcd" is rendered invalid after auth is enabled via initACL configs.

A potential alternative to not merging this change is to configure terraform tfvars with something like

eco_init_acl_users = [
...
  {
    name = "etcd"
    roles = ["root"]
    password = ""
  }
]

However, this is not very obvious and people might expect their client tls certs to be working after auth is enabled.

[Quentin-M]

Thank you very much @hvgirish!