refactor(bootstrap): nest network fields under system.network to match main project schema

templates/xen.cfg.j2

@@ -10,7 +10,7 @@ disk = [
   '{{ boot_iso }},,hdc,cdrom'{% if rhel_iso is defined and rhel_iso | length > 0 %}, '{{ rhel_iso }},,hdd,cdrom'{% endif %}
 {%- endif -%}
 ]
-vif = [ 'bridge={{ system_cfg.network }},model=e1000' ]
+vif = [ 'bridge={{ system_cfg.network.bridge }},model=e1000' ]
 boot = "{{ 'dc' if xen_installer_media_enabled | bool else 'c' }}"
 on_crash = "preserve"
 on_poweroff = "destroy"