ci-dockerslaves-cleanup: Run every 2 hours

Running hourly is really too frequently, such that even older builds expire
too soon.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I7b0c298ce3dbc043d8a17104ce6d8248785cb5dd
diff --git a/ci-dockerslaves-cleanup.yaml b/ci-dockerslaves-cleanup.yaml
index ab3a811..9ae9df9 100644
--- a/ci-dockerslaves-cleanup.yaml
+++ b/ci-dockerslaves-cleanup.yaml
@@ -24,7 +24,7 @@
     concurrent: false
     display-name: 'CI docker cleanup and image update'
     triggers:
-        - timed : '@hourly'
+        - timed : 'H */2 * * *'
     axes:
         - axis:
             type: slave