Add target_helper to backend_defaults

It seems when using Cinder backends, the value inside backend_defaults
in used which can default to the wrong value.

Change-Id: I771a2655545d0d006c882584657d97814e2fa8e1

templates/cinder.conf.j2

@@ -101,6 +101,9 @@ enabled_backends={% for backend in cinder_backends|dictsort %}{{ backend.0 }}{%
 {% endfor %}
 {% endif %}
 
+[backend_defaults]
+target_helper = {{ cinder_target_helper }}
+
 [database]
 connection = mysql+pymysql://{{ cinder_galera_user }}:{{ cinder_container_mysql_password }}@{{ cinder_galera_address }}/{{ cinder_galera_database }}?charset=utf8{% if cinder_galera_use_ssl | bool %}&ssl_ca={{ cinder_galera_ssl_ca_cert }}{% endif %}