[stats] Remove structure.clusters field from csv exporter of clusters information

[Andrey Sledzinskiy]

We have next field in csv exporter of clusters - structure.clusters
It is exported as empty because 'clusters' in installation structure is a dict with all cluster info that is parsed into separate fields
So I see no sense in keeping this field in exporter

[Andrey Sledzinskiy (asledzinskiy)]

Also installed_plugins field should be removed because we have separate report for them

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/167976

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/167976
Committed: https://git.openstack.org/cgit/stackforge/fuel-stats/commit/?id=0d7fd31a44b5f4e889abbbe83977dd83d2aa514d
Submitter: Jenkins
Branch: master

commit 0d7fd31a44b5f4e889abbbe83977dd83d2aa514d
Author: Alexander Kislitsky <email address hidden>
Date: Thu Mar 26 14:56:55 2015 +0300

    Unrequired columns removed from cluster CSV report

    Empty column clusters removed from clusters CSV report.
    We have separate report for plugins, so installed_plugins column
    is also removed from clusters CSV report.
    Sideffect with skeletons modification fixed.
    Clusters keys paths test fixed.

    Change-Id: I2f030390e8f687d87dab718d8c314edba5b6cc1f
    Closes-Bug: #1434819