Don't stop docker containers from supervisor

ATTENTION: this solution conflicts with
https://review.openstack.org/112077

- Use SIGTERM instead of SIGINT for dhcrelay_monitor supervisor
  service, because we have "sleep 30" loop in the monitor script,
  so it will handle SIGINT only when sleep ends.
- Send KILL instead of INT to "dockerctl attach" process to make
  sure we don't stop any container during supervisor shutdown or
  restart. This should make supervisor restart much faster and
  it's safe for upgrade process since we're not shutting down any
  containers. Supervisor only monitors them and brings them up,
  but does not stop them.

Change-Id: I08b50cf6f6ca7a93f446d07096ad25909ff60893
Related-bug: #1350764
Merge "Don't stop docker containers from supervisor"