Convert host value from unicode to a string.

The memcached API expects string format, not unicode.
Convert the host value to a string before creating
the cache_key and passing it to memcached.

Fixes: bug #1158958
Change-Id: I26e86b7fb61b4b3380a14686bd2faa9f58017999

nova/api/openstack/compute/contrib/extended_availability_zone.py

@@ -37,7 +37,7 @@ def __init__(self):
         self.mc = memorycache.get_client()
 
     def _get_host_az(self, context, instance):
-        host = instance.get('host')
+        host = str(instance.get('host'))
         if not host:
             return None
         cache_key = "azcache-%s" % host