kill only listeners we know we don't need anymore

[gordon chung]

when re-balancing queues across agents, we blindly kill all existing listeners and then recreate the appropriate new set of listeners. it's entirely possible that some of the existing listeners will be consumed by current agent so we should be smarter when deciding what to kill

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/225270

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/225270
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=338e7a655d5d422dc9db878d2f0952549e54e711
Submitter: Jenkins
Branch: master

commit 338e7a655d5d422dc9db878d2f0952549e54e711
Author: gordon chung <email address hidden>
Date: Fri Sep 18 14:39:25 2015 -0400

    retain existing listeners on refresh

    there is an overhead to killing and recreating listeners. this patch
    keeps the pipeline listeners we already have after rebalancing
    rather than blindly killing and recreating all of them.

    Change-Id: Ic7c23fd7649ca0b828cc4266582163bd326c2b80
    Closes-Bug: #1496459