software-properties-gtk should not disable security.ubuntu.com mirror to local one

[Norbert]

Steps to reproduce:
1. Install Ubuntu without any changes in /etc/apt/sources.list
(ensure that is has http://security.ubuntu.com/ubuntu

$ grep http://security.ubuntu.com/ubuntu /etc/apt/sources.list | grep -v ^#
deb http://security.ubuntu.com/ubuntu xenial-security main restricted
deb http://security.ubuntu.com/ubuntu xenial-security universe
deb http://security.ubuntu.com/ubuntu xenial-security multiverse
$

).
2. Launch `software-properties-gtk` and select local mirror, for example Switzerland mirror.switch.ch (or *Select Best Server*), *Reload* lists and check again:

$grep http://security.ubuntu.com/ubuntu /etc/apt/sources.list | grep -v ^#
$ # returns nothing

Notes:
* I discovered this while upgrading my machines to fix Meltdown vulnerability (as bug 1741934 and kernels 4.4.0-108 and 4.4.0-109). Only one my machine was upgraded to 109. It used *Main server* and http://security.ubuntu.com/ubuntu.
* All releases seems to be affected by this mirror-problem.
* Setting back to *Main server* does not return http://security.ubuntu.com/ubuntu to sources.list.
* This clearly does not correspond to the official documentation ( https://wiki.ubuntu.com/SecurityTeam/FAQ#What_repositories_and_pockets_should_I_use_to_make_sure_my_systems_are_up_to_date.3F )

Expected results:
* Ubuntu must save http://security.ubuntu.com/ubuntu mirror in /etc/apt/sources.list for really fast fixing of critical bugs

Actual results:
* Ubuntu switches all mirrors and disables http://security.ubuntu.com/ubuntu .

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: software-properties-gtk 0.96.20.7
ProcVersionSignature: Ubuntu 4.4.0-104.127-generic 4.4.98
Uname: Linux 4.4.0-104-generic x86_64
ApportVersion: 2.20.1-0ubuntu2.15
Architecture: amd64
CurrentDesktop: Unity
Date: Wed Jan 10 20:57:55 2018
InstallationDate: Installed on 2018-01-04 (6 days ago)
InstallationMedia: Ubuntu 16.04 LTS "Xenial Xerus" - Release amd64 (20160420.1)
PackageArchitecture: all
SourcePackage: software-properties
UpgradeStatus: No upgrade log present (probably fresh install)