tf-sync-repos.sh: force update to GitHub

1 Fixed an error when revising a tag.
    The tag had already been pushed to GitHub,
    so we couldn't push the revised tag without force.
    Solved the issue by using the '--force' option.
2. Increase the num-to-keep to 30

Signed-off-by: Arthur She <arthur.she@linaro.org>
Change-Id: I358517c97167a7e1ffc8521de3cfbc723ce26fa3
diff --git a/tf-sync-github-repos.yaml b/tf-sync-github-repos.yaml
index 7a67559..292749c 100644
--- a/tf-sync-github-repos.yaml
+++ b/tf-sync-github-repos.yaml
@@ -17,7 +17,7 @@
         !include-raw: ci/tf-sync-repos.sh
     properties:
     - build-discarder:
-        num-to-keep: 10
+        num-to-keep: 30
     publishers:
     - email:
         recipients: 'arthur.she@linaro.org paul.sokolovsky@linaro.org'