browbeat_user: "{{ ansible_user | default('stack') }}" in
install/group_vars/all.yml would lead to an infinite loop if not
run through ansible, i.e CI where we use ansible to install browbeat
This fix ensures that zuul uses another file to load up vars, and
thus browbeat can be installed manually as well.
Change-Id: Ieaacaba2c701013969fb10e689cf7d930afab934
be4e81c1de