Corrected the config file to use when stopping neutron-openvswitch-agent

The use of file ml12_conf.ini has been deprecated, replaced by /etc/neutron/plugins/ml2/openvswitch_agent.ini.

The command to cleanup the agent still references the old file.  Just fix the filename

https: //bugs.launchpad.net/kolla-ansible/+bug/1982222
Change-Id: I0fe7f68eda55e0c7d9960016bba74f5ba1ae223e