Allow None for ticks_between_runs when configuring periodic tasks

[Davanum Srinivas (DIMS)]

This bug was noticed when we set image_cache_manager_interval=0 as default. a review of the code shows that setting ticks_between_runs to 0 will end up executing the periodic_task for every tick. We need to allow setting image_cache_manager_interval to None (or -1?) so that nova/manager.py can entirely skip adding this task to its list of tasks to run.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/17076

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/17076
Committed: http://github.com/openstack/nova/commit/27868ddf260286d86c8e048934ffc7573d729702
Submitter: Jenkins
Branch: master

commit 27868ddf260286d86c8e048934ffc7573d729702
Author: Davanum Srinivas <email address hidden>
Date: Wed Nov 28 15:35:30 2012 -0500

    Don't run the periodic task if ticks_between_runs is below zero

    if the user sets say image_cache_manager_interval=-1, then
    don't add the task to the list of periodic tasks

    remove extra braces that i had added

    DocImpact

    Fixes LP #1084232

    Change-Id: Ieecd67ddbc70b815a88f40e72ca2899787d75988