opt_out_id can produce unicode keys

Riak requires string keys.

```
  File "/mnt/praekelt/vumi-go/go/vumitools/opt_out/models.py", line 44, in get_opt_out
    return self.opt_outs.load(self.opt_out_id(addr_type, addr_value))
  File "/mnt/praekelt/vumi-go/ve/src/vumi/vumi/persist/model.py", line 731, in load
    return self._modelcls.load(self._manager, key)
  File "/mnt/praekelt/vumi-go/ve/src/vumi/vumi/persist/model.py", line 268, in load
    return manager.load(cls, key, result=result)
  File "/mnt/praekelt/vumi-go/ve/src/vumi/vumi/persist/txriak_manager.py", line 104, in load
    riak_object = self.riak_object(modelcls, key, result)
  File "/mnt/praekelt/vumi-go/ve/src/vumi/vumi/persist/txriak_manager.py", line 72, in riak_object
    riak_object = RiakObject(self.client, bucket, key)
  File "/mnt/praekelt/vumi-go/ve/local/lib/python2.7/site-packages/riakasaurus/riak_object.py", line 52, in __init__
    raise TypeError('Unicode keys are not supported.')
exceptions.TypeError: Unicode keys are not supported.
```