No need to call pollster default discovery if resource is found through pipeline

[Lianhao Lu]

In the current PollingTask.poll_and_publish(), we use the following flow to get the candidate resources for the pollsters to poll:

step 1: call pollster's default discovery to get pollster_resources

step 2: get source_resources from discovery defined in pipeline

step 3: set the candidate_resources as:
            candidate_res = (source_resources or pollster_resources)

If we've found source_resources from step2, this is just a waste of time for step1. We need to adjust the flow.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/168251

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/168251
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=9efa08d4a322e6331b7b48b3a04b1c0636ed68d3
Submitter: Jenkins
Branch: master

commit 9efa08d4a322e6331b7b48b3a04b1c0636ed68d3
Author: Lianhao Lu <email address hidden>
Date: Fri Mar 27 15:07:02 2015 +0800

    Optimize the flow of getting pollster resources

    No need to call pollster's default discovery to get the resources if the
    pipeline has already provided them.

    Change-Id: Ib8959498b9dfd848087484d3348578733b790c29
    Closes-Bug: #1437175