NamespacesTest.testGetNamespaces fails sometimes

I've seen it failing just once on Travis build:

```
testGetNamespaces(com.yahoo.pulsar.broker.admin.NamespacesTest)  Time elapsed: 0.025 sec  <<< FAILURE!
java.lang.AssertionError: should have failed
    at com.yahoo.pulsar.broker.admin.NamespacesTest.testGetNamespaces(NamespacesTest.java:235)
```
