Read the DB name from the mongodb_uri

After this change, the mongodb_uri will need to include the DB name as described here: http://docs.mongodb.org/manual/reference/connection-string/

@haplo @thomasrockhu @rmariad can you guys take a look at this?
Yeah, this is what I get on my tests:
```python
>>> pymongo.MongoClient('mongodb://john:doe@host1,host2,host3/okapi?replicaSet=myreplica')           
Traceback (most recent call last):  
  File "<stdin>", line 1, in <module>  
  File "/home/lorenzo/.virtualenvs/okapiweb/local/lib/python2.7/site-packages/pymongo/mongo_client.py", line 386, in __init__  
    raise ConfigurationError(str(exc))  
pymongo.errors.ConfigurationError: command SON([('authenticate', 1), ('user', u'john'), ('nonce', u'2d115cee554acfa6'), ('key', u'flkasjfasdklfjaxsdfjasdfjafjdkfdjasfdskafasdf')]) failed: auth fails
```