Allow lock backup tasks

This patch allow us to lock backup tasks when processing.
So same task will be only process one time.
diff --git a/requirements.txt b/requirements.txt
index bc61474..dc53a3c 100755
--- a/requirements.txt
+++ b/requirements.txt
@@ -15,5 +15,6 @@
 openstacksdk>0.28.0
 pymysql
 parse
+tooz>=2.7.1 # Apache-2.0
 # email
 # smtplib