Change default for [kafka]num_processors to 1

Currently the default for [kafka]num_processors is 0. This can lead to
the situation where monasca-persister starts but nothing happens
because no processes are started.
This can happen very easy in the default configuration where the
"num_processors" value in the different
sections ([kafka_alarm_history], [kafka_events], [kafka_metrics]) is
set to:

num_processors = $kafka.num_processors

which means that in the default case, 0 processes are started.
This is fixed now because the default for [kafka]num_processors is now
1.

Change-Id: I2c8ed56bbe91648543044201d7b6320f88a724c7

monasca_persister/conf/kafka_common.py

@@ -42,7 +42,7 @@ kafka_common_opts = [
                default=32768),
     cfg.IntOpt('num_processors',
                help='Number of processes spawned by persister',
-               default=0)
+               default=1)
 ]
 
 kafka_common_group = cfg.OptGroup(name='kafka',

monasca_persister/persister.py

@@ -102,6 +102,10 @@ def prepare_processes(conf, repo_driver):
         for proc in range(0, conf.num_processors):
             processors.append(multiprocessing.Process(
                 target=start_process, args=(repository, conf)))
+    else:
+        LOG.warning("Number of processors (num_processors) is {}".format(
+            conf.num_processors))
+
 
 def main():
     """Start persister."""