Make tempest-slow run on stable branch also

tempest-slow has been added for stable branches also
- I5cb829beebcc59de682cba927f5b01793ec70d00

let's make this job definition to allow to run on stable branches also.

Change-Id: Ie5492603a12ca4ae56baa744bf650ae4d8036256
diff --git a/.zuul.yaml b/.zuul.yaml
index fb775f5..00dd92e 100644
--- a/.zuul.yaml
+++ b/.zuul.yaml
@@ -279,8 +279,6 @@
 - job:
     name: tempest-slow
     parent: tempest-multinode-full
-    branches:
-      - master
     description: |
       This multinode integration job will run all the tests tagged as slow.
       It enables the lvm multibackend setup to cover few scenario tests.