centos-release packages revert upstream repos during upgrade to MOS 9.2

[Anton Chevychalov]

Background
==========
1. centos-release package has a number of upstream repos configs inside it.
2. During installation from ISO that configs were wiped (see fuel-main repo)
3. In 9.2 we started to sync packages with origin CentOS (see mos-release package).

Bug Description
===============
Enabled upstream CentoOS repos configs appear in /etc/yum.repos.d.

Reproduce Step
==============
1. Install MOS 9.0.
2. Upgrade to MOS 9.2
3. Issue "yum repolist enabled"

Expected Result
===============
There are no "base", "extras" and "updates" repos.

Actual Result
=============
There are "base", "extras" and "updates" repos.

# yum repolist enabled
----------------------
Loaded plugins: fastestmirror, priorities
Loading mirror speeds from cached hostfile
 * base: mirrors.ocf.berkeley.edu
 * extras: centos.mirrors.hoobly.com
 * mos9.0-security: mirror.seed-us1.fuel-infra.org
 * mos9.0-updates: mirror.seed-us1.fuel-infra.org
 * mos92-updates: mirror.seed-us1.fuel-infra.org
 * updates: mirror.pac-12.org

Possible way to fix
===================
Add disabling of upstream repos into upgrade scripts.

[Fuel Devops McRobotson (fuel-devops-robot)]

Fix proposed to branch: 9.0/mitaka
Change author: Anton Chevychalov <email address hidden>
Review: https://review.fuel-infra.org/32422

[Fuel Devops McRobotson (fuel-devops-robot)]

Reviewed: https://review.fuel-infra.org/32422
Submitter: Pkgs Jenkins <email address hidden>
Branch: 9.0/mitaka

Commit: 99cd9838b1c70617fe2ae2a254d558ed9f7c8aca
Author: Anton Chevychalov <email address hidden>
Date: Wed Mar 22 12:27:06 2017

Disable upstream centos repos during upgrade

Enabled CentOS upstream repos appear after update to 9.2.
That fix disable it twice (before installation and after).
For clarification see LP bug.

Closes-Bug: #1673428

Change-Id: I67f0d840a3023813ec6742d524d5c5d022ef9857

[Anton Chevychalov (achevychalov)]

Bug was fixed for two different scenarios:

First one:

* Install 9.0
* Update to 9.2
* Bug is there
* Update to 9.2 mu1
* Bug has been fixed

Second one:

* Install 9.0
* Update to 9.2 mu1
* Bug has been fixed

There are two separate entry points in code for each scenario. So both of them have to be tested.

[TatyanaGladysheva (tgladysheva)]

Verified on 9.2 mu1 updates using both scenarios from comment 3.

Actual Result:
There are no "base", "extras" and "updates" repos.