[systest] Re-factor methods which modify astute.yaml

[Artem Panchenko]

In system tests we modify a couple of settings stored in /etc/fuel/astute.yaml:

https://github.com/stackforge/fuel-qa/blob/master/fuelweb_test/helpers/fuel_actions.py#L118-L120
https://github.com/stackforge/fuel-qa/blob/master/fuelweb_test/helpers/fuel_actions.py#L135

In order to avoid issues like this bug #1454988 we should avoid using of 'sed' for that and use yaml load/dump. For example we have a method which loads settings from astute.yaml file:

https://github.com/stackforge/fuel-qa/blob/master/fuelweb_test/models/environment.py#L449

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/248499

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/248499
Committed: https://git.openstack.org/cgit/openstack/fuel-qa/commit/?id=1b67512152518fe43bdb2124d5ff9d3c5bdac3b3
Submitter: Jenkins
Branch: master

commit 1b67512152518fe43bdb2124d5ff9d3c5bdac3b3
Author: Artem Panchenko <email address hidden>
Date: Mon Nov 23 00:37:34 2015 +0200

    Use YAML methods for changing Fuel 'astute.yaml'

    Use load/dump methods for changing content of 'astute.yaml'
    while deploying Fuel master node.

    Change-Id: I71cb042927b912dd7b610cbaadca48919dafc47c
    Closes-bug: #1455015