Kill the vrrp orphan process when (re)spawn keepalived

Kill the vrrp orphan process when (re)spawn keepalived

When keepalived crashed unexpectedly, the vrrp process that
it associates with will be orphan process. This will make
the VIP unable to migrate to the router in the same host.
Also, neutron code is not able to respawn the keepalived
process, because keepalived thinks itself is still running,
according to [1-3]. As a result, neutron will report respawning
keepalived all the time. Restart l3-agent will not help.

This patch will check and delete the orphan vrrp process
if there is any, in the processmonitor of l3 agent.

More details can be found in the bug description and comments.

[1] https://goo.gl/W3GL9I
[2] https://goo.gl/F0Ixfb
[3] https://goo.gl/dUqhTo

Change-Id: Ia1759ed1365b845d404686a8cd25f882cce35caf
Closes-Bug: #1511311