Systemd processes use a default KillMode of 'control-group'
which causes all other processes spawned during execution to be
killed on service stop. Neutron expects the keepalived processes
it starts to remain running in order to prevent data-plane
interruptions for HA routers.
This change switches the systemd KillMode to process in order to
prevent this issue. In doing so we also have to clean up
non-keepalived processes started by neutron so that upon restart
everything is running from the latest virtualenv which may have
changed during an upgrade.
Change-Id: I958fda17e6207553466d8a7512e35c30b122c22c
Closes-Bug: #1846198
Depends-On: https://review.opendev.org/771770