Fix neutron-metadata-agent to use provided CA for Nova metadata

Change-Id: I48148c56079a2998a8f8606ecd1f185f68733a6d
Partially-implements: blueprint custom-cacerts

ansible/roles/neutron/templates/metadata_agent.ini.j2

 # metadata_agent.ini
 [DEFAULT]
+auth_ca_cert = {{ openstack_cacert | default(omit) }}
 nova_metadata_host = {{ nova_internal_fqdn }}
 nova_metadata_port = {{ nova_metadata_port }}
 metadata_proxy_shared_secret = {{ metadata_secret }}