undercloud-post: Discard CA usage if not set
undercloud-post: Discard CA usage if not set

If there is nothing set for the CA, discard the value. This way we can
ignore this value when using trusted certificates.

Change-Id: Ia0085c43fe9468cd1827f6e4ae39f48ce0e398b6
Related-Bug: #1785059

openstack/tripleo-heat-templates,dprince/tripleo-heat-templates,dprince/tripleo-heat-templates,openstack/tripleo-heat-templates