Add the api works num for watcher

Number of workers for Watcher api service can not be configure, this
patch to fix it.

refer to https://github.com/openstack/watcher/blob/master/watcher/conf/api.py#L43

Change-Id: I2763b8e225ceb4b8e067d53b8d5f0ffba31563a6

ansible/roles/watcher/templates/watcher.conf.j2

@@ -9,6 +9,7 @@ transport_url = rabbit://{% for host in groups['rabbitmq'] %}{{ rabbitmq_user }}
 [api]
 host = {{ api_interface_address }}
 port = {{ watcher_api_port }}
+workers = {{ openstack_service_workers }}
 {% endif %}
 
 [database]