Nailgun statistics module needs refactoring

[Artem Roma]

Statistics collecting feature performs two separate logical tasks: collecting of the statistics of the fuel infrastructure using and collecting info about oswl resources. As for now code for both of that tasks are mixed in one package (nailgun.statistics) which perplexing structure make it hard (for example) to reuse same (utility) code in both parts without additional refactoring effort. For simplifying of managing of the statistics' codebase we should create subpackages where code coupled with same logic will be placed and move utility infrastructure (helper functions, custom errors definition etc.) on higher level.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/169800

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/169800
Committed: https://git.openstack.org/cgit/stackforge/fuel-web/commit/?id=4603980b81ba6bec952b31cb7255de2ceb49720a
Submitter: Jenkins
Branch: master

commit 4603980b81ba6bec952b31cb7255de2ceb49720a
Author: Artem Roma <email address hidden>
Date: Wed Apr 1 18:09:58 2015 +0300

    Refactor nailgun.statistics package

    Two subpackages (for statistic collecting of fuel infrastructure using
    and collecting of oswl info) created in order to separate code dedicated
    to logically different functions. Utility (universal utility functions
    and custom errors description) infrastructure left on
    statistics package level with possibility of reusing in subpackages'
    code. Tests refactored considering current separation.

    Change-Id: I89509d5fcd224276c3cd6c95353cea0ebca2162e
    Closes-Bug: #1439268