Use random exponential delay

this is much more suited for resolving contention when
accessing a (lockable) shared resource.

Change-Id: I93abdbf8a9532af1aec7ad15a6f384beb9fc3ff9
(cherry picked from commit 56f6e583d1)

heat/objects/resource.py

@@ -41,7 +41,7 @@ LOG = logging.getLogger(__name__)
 def retry_on_conflict(func):
     wrapper = tenacity.retry(
         stop=tenacity.stop_after_attempt(11),
-        wait=tenacity.wait_random(max=2),
+        wait=tenacity.wait_random_exponential(multiplier=0.5, max=60),
         retry=tenacity.retry_if_exception_type(
             exception.ConcurrentTransaction),
         reraise=True)