Fix AAE on/off detection

riak_kv/anti_entropy takes a value of the form {on|off, Opts}, not on
off.

This fixes basho/riak_kv#988

Without this fix, you would see messages like this:

```
2014-06-17 12:00:05.829 [warning] <0.384.0>@riak_kv_ensembles:maybe_aae_configuration_valid:168 Strong consistency has been enabled without AAE -- all consistent operations will timeout until AAE is enabled.
```

Even with active anti-entropy enabled. Now, these messages are only shown if anti_entropy is set to passive.  When it is active or active-debug, they will not show up