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/ci/tf-sync-repos.sh b/ci/tf-sync-repos.sh
index dca61b5..011af75 100755
--- a/ci/tf-sync-repos.sh
+++ b/ci/tf-sync-repos.sh
@@ -84,5 +84,5 @@
     echo -e "\nFetching Gerrit ${GERRIT_PROJECT}:${gerrit_branch}\n"
     git -C "${_workdir}" fetch --tags "${gerrit_repo}" "${gerrit_branch}"
     echo -e "\nSyncing up to GitHub ${gh_repo}:${gh_branch}\n"
-    git -C "${_workdir}" push --tags "${gh_remote}" FETCH_HEAD:refs/heads/"${gh_branch}"
+    git -C "${_workdir}" push --force --tags "${gh_remote}" FETCH_HEAD:refs/heads/"${gh_branch}"
 done
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'