Trigger nova client may cause large number of requests at same time

Fix proposed to branch: master
Review: https://review.openstack.org/148404

Reviewed: https://review.openstack.org/148404
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=3b38a1e27c75a788446aa2c6b9f1418110e99c71
Submitter: Jenkins
Branch: master

commit 3b38a1e27c75a788446aa2c6b9f1418110e99c71
Author: mizeng <email address hidden>
Date: Tue Jan 20 10:02:36 2015 +0800

    Shuffle agents to send request

    For each polling interval, Ceilometer compute agent will trigger
    nova client directly to acquire the list of VMs.

    However, there's a potential risk that the workload for nova client
    will increases proportional to the number of Ceilometer compute
    agents. In other word, there's a possibility that thousands of
    ceilometer compute agents will call nova client in a very short time
    period which may lead large number of requests to nova client in
    short time. As a result, the nova client can not handle so many
    requests in short time, and ceilometer compute agent can't fetch
    metrics.

    So we here shuffle agents to start polling task in order to fluff up
    the time of sending request to nova or other components to avoid
    large number of requests in short time. This would be more
    performance effective and reliable.

    DocImpact

    Change-Id: I12e3f104fc92fe15adc05e2b981627f31ee5bfaa
    Closes-bug: #1412613