Use gunicorn instead of werkzeug

Flask use werkzeug as default WSGI container, but it cannot support
production requirement and leads to crash in some cases. So use
gunicorn instead of it.

Change-Id: I2e542ab79c755c2407ae695a6de29c063233269b
Closes-Bug: #1641242

requirements.txt

@@ -11,4 +11,4 @@ python-dateutil==2.5.3
 pytz==2016.7
 requests==2.11.1
 six==1.10.0
-Werkzeug==0.11.11
+gunicorn==19.6.0

setup.cfg

@@ -52,4 +52,4 @@ source-dir = releasenotes/source
 
 [entry_points]
 console_scripts =
-    valence = valence.run:main
+    valence = valence.cmd.api:main

valence/cmd/api.py

@@ -14,16 +14,42 @@
 #    under the License.
 
 import logging
+
+import gunicorn.app.base
+from gunicorn.six import iteritems
+
 from valence.api.route import app as application
 from valence import config as cfg
 
+
 LOG = logging.getLogger(__name__)
 
 
+class StandaloneApplication(gunicorn.app.base.BaseApplication):
+
+    def __init__(self, app, options=None):
+        self.options = options or {}
+        self.application = app
+        super(StandaloneApplication, self).__init__()
+
+    def load_config(self):
+        config = dict([(key, value) for key, value in iteritems(self.options)
+                       if key in self.cfg.settings and value is not None])
+        for key, value in iteritems(config):
+            self.cfg.set(key.lower(), value)
+
+    def load(self):
+        return self.application
+
+
 def main():
-    application.run(host=cfg.bind_host, port=cfg.bind_port, debug=cfg.debug)
+    options = {
+        'bind': '%s:%s' % (cfg.bind_host, cfg.bind_port)
+    }
+    StandaloneApplication(application, options).run()
     LOG.info(("Valence Server on http://%(host)s:%(port)s"),
              {'host': cfg.bind_host, 'port': cfg.bind_port})
 
+
 if __name__ == '__main__':
     main()