[nailgun] Refactor attributes generation code

[Aleksey Kasatkin]

Now there are two methods for attributes traversal: traverse() and generate_editables(). They do almost the same things so let's DRY this.
Generator methods in AttributesGenerator() are to be reworked also.

Tests for generators are required also.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/133960

[OpenStack Infra (hudson-openstack)]

Change abandoned by Ivan Kliuk (<email address hidden>) on branch: master
Review: https://review.openstack.org/133960
Reason: The functionality has been implemented in patch-set #166226.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/166226
Committed: https://git.openstack.org/cgit/stackforge/fuel-web/commit/?id=a80c2709015cc19eda96d5939f940e5ada674d20
Submitter: Jenkins
Branch: master

commit a80c2709015cc19eda96d5939f940e5ada674d20
Author: Igor Kalnitsky <email address hidden>
Date: Fri Mar 20 12:21:59 2015 +0200

    Use traverse for generating repo_setup

    Currently we're using Jinja2 template engine for generating repo_setup
    data in openstack.yaml. That's bad, because it requires to do so in
    fuel_upgrade script and doesn't support proper "OpenStack version"
    replacement.

    Since now, we're using our traversal mechanism for generating cluster
    attributes.

    Closes-Bug: #1434494
    Closes-Bug: #1390017

    Change-Id: Ia03308514e9e6b9b6ed8d14d0a156e14314b21e2