pulling "master" from repo init always on `xos-unit` test jobs as some projects have different branches

Change-Id: I1be86b3154da3aa08345b6fd6d4d3c2151c14fa3
diff --git a/jjb/xos-unit.yaml b/jjb/xos-unit.yaml
index b619569..b474971 100644
--- a/jjb/xos-unit.yaml
+++ b/jjb/xos-unit.yaml
@@ -29,7 +29,7 @@
     scm:
       - cord-infra-gerrit-repo-scm:
           manifest-url: '{gerrit-server-url}/{cord-repo-manifest}'
-          branch: '$GERRIT_BRANCH'
+          branch: 'master'
           destination-dir: 'cord'
 
     node: 'ubuntu16.04-basebuild-1c-2g'
@@ -58,4 +58,3 @@
                 healthy: 50
                 unhealthy: 0
                 failing: 0
-