Make clusterctl run only on ubuntu

Change-Id: I2c6e2366498cf23ec0fc157738e2e730b65b4f52
diff --git a/jjb/metal3/projects.yml b/jjb/metal3/projects.yml
index c410dd1..7adabeb 100644
--- a/jjb/metal3/projects.yml
+++ b/jjb/metal3/projects.yml
@@ -451,6 +451,8 @@
             target_branches:
             - 'release-1.4'
             - 'release-0.3'
+        exclude:
+        - image_os: centos
     - keep_capm3_e2e_integration_tests:
         capm3_target_branch:
         - main: