Tests: _river should have only one shard one replica

By the way, though a `_river` index should have only one shard and one replica, our test infra builds a random number of shards. We should fix it by forcing the number of shards and replicas before starting the test. Seed: `D5BE6F4EED5380F7` for example.

```sh
mvn clean test -Dtests.seed=D5BE6F4EED5380F7
```

Closes #38.
Tests: wait for bulk before closing the test

When closing a test, our test infra removes created indices.

But as the river is still processing documents, the bulk processor contains remaining document which have not been indexed yet.

When the river stops, it fires a `close()` call on the bulk processor.
This call creates again the index. Test framework throw an exception as the mock directory wrapper can not close due to write operations in progress.

Logs:

```
  1> [2014-10-09 11:59:40,965][INFO ][cluster.metadata         ] [node_s0] [_river] deleting index
  1> [2014-10-09 11:59:40,982][INFO ][test.store               ] [node_s1] [_river][5] Shard state before potentially flushing is STARTED
  1> [2014-10-09 11:59:41,084][INFO ][test.store               ] [node_s3] [_river][0] Shard state before potentially flushing is STARTED
  1> [2014-10-09 11:59:41,115][INFO ][test.store               ] [node_s2] [_river][6] Shard state before potentially flushing is STARTED
  1> [2014-10-09 11:59:41,943][INFO ][river.wikipedia          ] [node_s1] [wikipedia][wikipedia] closing wikipedia river
  1> [2014-10-09 11:59:42,693][WARN ][river.wikipedia          ] [node_s1] [wikipedia][wikipedia] river was closing while processing wikipedia page [39]/[Aube (département)
  1>     ]. Operation skipped.
  1> [2014-10-09 11:59:42,768][INFO ][river.wikipedia          ] [WikipediaRiverTest#testWikipediaRiverFrench]: cleaning up after test
  1> [2014-10-09 11:59:42,769][INFO ][cluster.metadata         ] [node_s0] [wikipedia] deleting index
  1> [2014-10-09 11:59:43,587][WARN ][river.wikipedia          ] There was failures while executing bulk
  1> [2014-10-09 11:59:44,971][INFO ][cluster.metadata         ] [node_s0] [wikipedia] creating index, cause [auto(bulk api)], shards [9]/[2], mappings [_default_]
  1> [2014-10-09 11:59:45,805][INFO ][cluster.metadata         ] [node_s0] [wikipedia] update_mapping [page] (dynamic)
  1> [2014-10-09 12:00:00,466][INFO ][test.store               ] [node_s2] [wikipedia][0] MockDirectoryWrapper#close() threw exception
  1> java.lang.RuntimeException: MockDirectoryWrapper: cannot close: there are still open locks: {write.lock=java.lang.RuntimeException: lock "write.lock" was not released}
  1> 	at org.apache.lucene.store.MockDirectoryWrapper.close(MockDirectoryWrapper.java:709)
  1> 	at org.elasticsearch.test.store.MockDirectoryHelper$ElasticsearchMockDirectoryWrapper.close(MockDirectoryHelper.java:140)
  1> 	at org.elasticsearch.test.hamcrest.ElasticsearchAssertions.assertAllFilesClosed(ElasticsearchAssertions.java:667)
  1> 	at org.elasticsearch.test.TestCluster.assertAfterTest(TestCluster.java:76)
  1> 	at org.elasticsearch.test.ElasticsearchIntegrationTest.afterInternal(ElasticsearchIntegrationTest.java:603)
  1> 	at org.elasticsearch.test.ElasticsearchIntegrationTest.after(ElasticsearchIntegrationTest.java:1722)
```

Closes #37.