Use www-data user for bifrost nginx log directory

Bifrost uses the www-data user for nginx on debian/ubuntu, and nginx on
other platforms. Kolla-ansible currently uses the nginx user for all
platforms when creating a log directory. This change uses the www-data
user on debian/ubuntu when setting ownership of the bifrost nginx log
directory.

Change-Id: I142a158b6f2e8f8a46b684267f6fbb2a6e22a259
Closes-Bug: #1753750

ansible/roles/bifrost/tasks/bootstrap.yml

@@ -1,5 +1,8 @@
 ---
 - name: Ensure log directories exist
+  vars:
+    nginx_user: "{{ 'www-data' if is_debian else 'nginx' }}"
+    is_debian: "{{ kolla_base_distro in ['debian', 'ubuntu'] }}"
   command: >
      docker exec bifrost_deploy
      bash -c 'mkdir -p /var/log/kolla/ironic &&
@@ -7,7 +10,7 @@
      mkdir -p /var/log/kolla/ironic-inspector &&
      chown ironic:ironic /var/log/kolla/ironic-inspector &&
      mkdir -p /var/log/kolla/nginx &&
-     chown nginx:nginx /var/log/kolla/nginx'
+     chown {{ nginx_user }}:{{ nginx_user }} /var/log/kolla/nginx'
 
 - name: Bootstrap bifrost (this may take several minutes)
   command: >