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