Fix neutron agents restarted on ml2 config change

The following Neutron agents:
- neutron-metadata
- neutron-dhcp-agent
- neutron-l3-agent
- neutron-lbaasv2-agent
- neutron-vpn-agent
doesn't require ml2_conf.ini file.
ml2_conf.ini file is used to managed L2 network configuration.
This config doesn't have to be in DHCP, L3, metadata agents...

We should remove it to avoid restarting these agents in case of
ml2_conf.ini file modification.
Only neutron-server, neutron-openvswitch-agent,
neutron-linuxbridge-agent must be restarted.

Closes-Bug: #1677163
Change-Id: I0876b8a3845d1c2bccd996426a65df1a3a6f7085