Zuul cloner now defaults --branch to $ZUUL_BRANCH

Upstream changed zuul-cloner --branch argument to default to
$ZUUL_BRANCH.  So we no more need to hardcoded it.

Reference:
https://review.openstack.org/#/c/116098/

Change-Id: If67bb9cc2b3f17322831417db5e4cf4828a032fc