Add a /healthcheck URL

The /healthcheck is helpful for operators to setup zun-api
behind haproxy, or for doing monitoring.

Change-Id: Ibca14ebe606eb90e7b9af740f245604e97dd8cf1

etc/zun/api-paste.ini

@@ -1,4 +1,9 @@
-[pipeline:main]
+[composite:main]
+use = egg:Paste#urlmap
+/: zun_api_pipeline
+/healthcheck: healthcheck
+
+[pipeline:zun_api_pipeline]
 pipeline = cors request_id osprofiler authtoken api_v1
 
 [app:api_v1]
@@ -17,3 +22,8 @@ paste.filter_factory = oslo_middleware:RequestId.factory
 [filter:cors]
 paste.filter_factory =  oslo_middleware.cors:filter_factory
 oslo_config_project = zun
+
+[app:healthcheck]
+paste.app_factory = oslo_middleware:Healthcheck.app_factory
+backends = disable_by_file
+disable_by_file_path = /etc/zun/healthcheck_disable

etc/zun/zun-config-generator.conf

@@ -9,6 +9,7 @@ namespace = oslo.db
 namespace = oslo.log
 namespace = oslo.messaging
 namespace = oslo.middleware.cors
+namespace = oslo.middleware.healthcheck
 namespace = oslo.policy
 namespace = oslo.privsep
 namespace = oslo.service.periodic_task