Don't set mysql_service_name for bifrost

Bifrost will determine the name of the service for MySQL based on the OS
distro if mysql_service_name is not set. Setting it explicitly in
extra-vars was causing problems on systems that use mariadb.

Change-Id: I892b1c9cf636b9dfc1bacc92e1e2f0d066018260
Closes-Bug: #1753522

ansible/roles/bifrost/templates/bifrost.yml.j2

@@ -8,6 +8,5 @@ inspector_log_dir: "/var/log/kolla/ironic-inspector"
 # Nginx log directory.
 nginx_log_dir: "/var/log/kolla/nginx"
 
-mysql_service_name: mysql
 ansible_python_interpreter: /var/lib/kolla/venv/bin/python
 network_interface: "{{ bifrost_network_interface }}"