Multiple testing processes with xdist all try to create the same runt…

…ime keyspace (#125)

* Detect database exists exception
* Create the test keyspace not the runtime keyspace

django_cassandra_engine/base/creation.py

@@ -54,7 +54,7 @@ def create_test_db(self, verbosity=1, autoclobber=False, **kwargs):
         replication_factor = replication_opts.pop('replication_factor', 1)
 
         create_keyspace_simple(
-            self.connection.settings_dict['NAME'],
+            test_database_name,
             replication_factor,
             connections=[self.connection.alias])