Stop Neutron advanced service external processes

Some of Neutron advanced services (LBaaS, VPNaaS, ....) creates
external processes and they should be stopped in unstack.
This commit defines neutron_<XaaS>_stop functions for all services
and implements the cleanup logics if necessary.

Also cleanup_neutron removes netns used by LBaaS haproxy.

Change-Id: Ied3a2c374ffcb6b59ecaf1027fb6e6083eded2ae
Merge "Stop Neutron advanced service external processes"