Enable Process Monitor by default.

Enable Process Monitor by default.

Process monitor is enabled by default by this patch,
with a default 60 second monitoring interval, this
interval was calculated early in the development
process to scale to 1000s of processes with light load.
We believe it's important to have it enabled to get
user feedback as we release kilo.

Process monitor is sucessfully enabled and backported
to Red Hat D/S distributions from icehouse to juno
without any issue.

Specific process monitor functional tests provide
coverage, also keepalived checks that it can be
properly respawned.

We should follow up with dhcp and l3 agent functional
testing for killing and checking their processes
correctly respawned. Normal process start/stop is
already validated by other functional tests and tempest.

DocImpact

Change-Id: I85fe31bee30714148168a293eea29fa0a37f9701
Implements: blueprint agent-child-processes-status