Kolla Ansible sets kolla_{external,internal}_fqdn_cacert variables with
default values compatible with the use of `kolla-ansible certificates`.
However, when these variables are left unset in Kayobe, which is
generally the case when using trusted certificates, we end up with
openrc files setting OS_CACERT to a file that does not exist:
${KOLLA_CONFIG_PATH}/certificates/haproxy-ca.crt
Instead we allow null cacert variables to be passed to kolla-ansible,
which results in openrc files without the bogus OS_CACERT entry.
Change-Id: Ifa615888b6d8d54c9e6314fd90f3fc4872fc6e5a
Story: 2007516
Task: 39299