fix branch handling in release clone_repo function
fix branch handling in release clone_repo function

When no branch is specified, we don't want to include the '--branch'
argument to clone_repo.sh.

Change-Id: Ie9ee423835ba7ba6e8822748db29aeab5b1d31fc
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack-infra/project-config,openstack-infra/project-config