tf-a-lts-create-release: For sandbox run, push commit to a branch

If it's sandbox run, we still need to push commit to some branch,
because otherwise the tag won't be visible in Gerrit (due to ACL
model Gerrit uses: it resolve tag to a branch it belongs to, then
checks ACLs on branch to see if a user has access to it; no branch
== problems).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Ifa467c9b07951e3e9997fa0d687ea08f9e459144
1 file changed