Fix problem when using wsgi script to start

The calling of method prepare_service is missing when ceilometer-
api started using wsgi script that pbr generated, this patch fixed
this problem.

Change-Id: I2210f408b0d3cc8378d62f62edce4edbe8102dad
Closes-Bug: #1625918

ceilometer/api/app.py

@@ -24,6 +24,8 @@ import pecan
 from ceilometer.api import hooks
 from ceilometer.api import middleware
 
+from ceilometer import service
+
 LOG = log.getLogger(__name__)
 
 CONF = cfg.CONF
@@ -94,3 +96,8 @@ def load_app():
 
 def app_factory(global_config, **local_conf):
     return setup_app()
+
+
+def build_wsgi_app():
+    service.prepare_service()
+    return load_app()

setup.cfg

@@ -267,7 +267,7 @@ ceilometer.event.trait_plugin =
     timedelta = ceilometer.event.trait_plugins:TimedeltaPlugin
 
 wsgi_scripts =
-    ceilometer-api = ceilometer.api.app:load_app
+    ceilometer-api = ceilometer.api.app:build_wsgi_app
 
 console_scripts =
     ceilometer-polling = ceilometer.cmd.polling:main