feat: disable unneeded message-of-the-day sections

[mboersma]

Reason for Change:
Disables the "news" and "release upgrade" scripts that populate the traditional motd screen seen when logging into an AKS Engine master or node.

The news is often about Ubuntu-specific Kubernetes offerings (which are irrelevant since the context here is a running Azure k8s cluster), and upgrading to a new major Ubuntu release is a great way to break things. Let's not clutter the login screen with what amounts to bad advice.

Before:

Authorized uses only. All activity may be monitored and reported.
Welcome to Ubuntu 16.04.6 LTS (GNU/Linux 4.15.0-1071-azure x86_64)

 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/advantage

 * Kubernetes 1.18 GA is now available! See https://microk8s.io for docs or
   install it with:

     sudo snap install microk8s --channel=1.18 --classic

 * Multipass 1.1 adds proxy support for developers behind enterprise
   firewalls. Rapid prototyping for cloud operations just got easier.

     https://multipass.run/

2 packages can be updated.
0 updates are security updates.

New release '18.04.4 LTS' available.
Run 'do-release-upgrade' to upgrade to it.


*** System restart required ***

After:

Authorized uses only. All activity may be monitored and reported.
Welcome to Ubuntu 16.04.6 LTS (GNU/Linux 4.15.0-1071-azure x86_64)

 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/advantage

2 packages can be updated.
0 updates are security updates.


*** System restart required ***

chmod -x is the recommended way to disable parts of the motd functionality.

$ ls -al /etc/update-motd.d/
total 52
drwxr-xr-x   2 root root 4096 Mar 17 08:13 .
drwxr-xr-x 105 root root 4096 Mar 30 22:04 ..
-rwxr-xr-x   1 root root 1220 Oct 22  2015 00-header
-rwxr-xr-x   1 root root 1157 Sep 30  2019 10-help-text
-rw-r--r--   1 root root 4646 Sep 30  2019 50-motd-news
-rwxr-xr-x   1 root root   97 Dec  7  2018 90-updates-available
-rw-r--r--   1 root root  299 Oct  2 14:08 91-release-upgrade
-rwxr-xr-x   1 root root  165 Feb 17 11:39 92-unattended-upgrades
-rwxr-xr-x   1 root root  111 Oct  1  2018 97-overlayroot
-rwxr-xr-x   1 root root  142 Dec  7  2018 98-fsck-at-reboot
-rwxr-xr-x   1 root root  144 Dec  7  2018 98-reboot-required
-rwxr-xr-x   1 root root  604 Nov  5  2017 99-esm

Issue Fixed:

Requirements:

• uses conventional commit messages
• includes documentation
• adds unit tests
• tested upgrade from previous version

Notes:

[acs-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mboersma

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• OWNERS [mboersma]

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[codecov]

Codecov Report

Merging #3000 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3000   +/-   ##
=======================================
  Coverage   72.09%   72.09%           
=======================================
  Files         141      141           
  Lines       26252    26252           
=======================================
  Hits        18927    18927           
  Misses       6230     6230           
  Partials     1095     1095

[jackfrancis]

/lgtm