ceilometer-polling has no validation for namespace duplication

[Liusheng]

e.g. When use following command to launch a ceilometer polling service with duplicated namespace "compute", the service can be stated successfully, and the pollsters of compute namespace will be run twice at one time.

/usr/bin/python /usr/local/bin/ceilometer-polling --config-file=/etc/ceilometer/ceilometer.conf --polling-namespaces {compute,compute} --log-file /var/log/ceilometer/ceilometer-polling.log

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/161996

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/161996
Committed: https://git.openstack.org/cgit/openstack/ceilometer/commit/?id=b795b755aeccb09b21d50d97a1fcb5a78426a9ba
Submitter: Jenkins
Branch: master

commit b795b755aeccb09b21d50d97a1fcb5a78426a9ba
Author: liu-sheng <email address hidden>
Date: Fri Mar 6 10:56:37 2015 +0800

    Check the namespaces duplication for ceilometer-polling

    This change add validation for the namespaces duplication when launch
    a ceilometer-polling service with duplicated namespaces specified by
    --polling-namespaces option.

    Change-Id: Iacd80b77bda4ae16d0307ebed6aa9903204c94d2
    Closes-Bug: #1428476