Fix SAPQuotaEngine cache client initialization

The SAPQuotaEngine instance is created as a module variable
(nova.quota.QUOTAS), so the CONF might have not been initialized
at the point when SAPQuotaEngine.__init__ is called.

Similarily to how the self._driver is initialised, we are now
initializing the self._cache client outside of the constructor,
making sure the CONF is also initialized at that point.

This fixes the issues where DictCacheBackend was used by the
SAPQuotaEngine, instead of the configured Memcached backend.

Change-Id: Iceba3c7820f9c6b9255ef6e00a9f49ed4677c12f

Author: leust

nova/quota.py

@@ -1117,10 +1117,17 @@ class SAPQuotaEngine(QuotaEngine):
 
     def __init__(self, quota_driver=None, resources=None):
         self._original_resources = {}
-        expiration_time = CONF.quota.sap_resources_cache_time
-        self._cache = cache.get_client(expiration_time=expiration_time)
+        self.__cache = None
         super().__init__(resources=resources, quota_driver=quota_driver)
 
+    @property
+    def _cache(self):
+        if not self.__cache:
+            expiration_time = CONF.quota.sap_resources_cache_time
+            self.__cache = cache.get_client(
+                                    expiration_time=expiration_time)
+        return self.__cache
+
     @property
     def _resources(self):
         resources = self._cache.get(self._CACHE_KEY)