make interval optional in pipeline

[gordon chung]

the pipeline yaml file requires an interval to be defined. this option makes no sense in the context of notification agent and is actually ignored.

this attribute should be optional and be given a default in the case of polling agents.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/179732

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/179732
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=e27b11ae7438c63eed3c6e16810105cc6a5f89c1
Submitter: Jenkins
Branch: master

commit e27b11ae7438c63eed3c6e16810105cc6a5f89c1
Author: leizhang <email address hidden>
Date: Mon May 4 16:05:16 2015 +0800

    Make interval optional in pipeline

    Make interval in pipeline yaml file optional. If interval is not
    provided, a default value should be given.

    Change-Id: I557e66902c272331bd1e2c72d03d495f0924d573
    Closes-Bug: #1423957