Make auth_url lookup dynamic

If _get_auth_url() is run on heat-api startup, it can cause heat-api to crash
in situations where Keystone is not running, yet or temporarily unavailable.
This patch converts the auth_url attribute into a property method that is only
run when it is needed, thus preventing this race condition.

Change-Id: Ife6d9e51ea9647e4658105c016867efe769e5363
Closes-Bug: #1550284
(cherry picked from commit fe92268987)

heat/common/auth_url.py

@@ -28,7 +28,13 @@ class AuthUrlFilter(wsgi.Middleware):
     def __init__(self, app, conf):
         super(AuthUrlFilter, self).__init__(app)
         self.conf = conf
-        self.auth_url = self._get_auth_url()
+        self._auth_url = None
+
+    @property
+    def auth_url(self):
+        if not self._auth_url:
+            self._auth_url = self._get_auth_url()
+        return self._auth_url
 
     def _get_auth_url(self):
         if 'auth_uri' in self.conf: