Set correct linux distro for packer

The current packer variables list centos-7 instead of centos as the
configuration file name. Therefore the build matrix needs to be updated

Change-Id: I5e4ee8973bbf3f3f70f6ab1670f5c2620b69d23f
Signed-off-by: Andrew Grimberg <agrimberg@linuxfoundation.org>
diff --git a/jjb/ci-management/ci-management.yaml b/jjb/ci-management/ci-management.yaml
index 6bb4fea..57dd33f 100644
--- a/jjb/ci-management/ci-management.yaml
+++ b/jjb/ci-management/ci-management.yaml
@@ -9,7 +9,7 @@
       - '{project-name}-ci-jobs'
       - '{project-name}-packer-jobs':
           platforms:
-            - centos
+            - 'centos-7'
           templates:
             - basebuild