Remove 'container' argument from CentralNode, WorkerNode, RelayNode constructors.

[dceara]

As @mkalcok points out these arguments give the wrong impression that users can choose random names. That's not exactly true. Container names are either (partially) hardcoded in ovn-fake-multinode or can be changed via config. It's probably better to pass the config (and maybe an index or some other unique id) and do the container naming internally.

[dceara]

Originally reported at: #180 (comment)