chore: add system.conf with static JoinControllers configuration

[jackfrancis]

Reason for Change:

This configuration was recently removed because of systemd release notes announcing deprecation of JoinControllers starting with version 240:

https://lists.freedesktop.org/archives/systemd-devel/2018-December/041852.html

However, Ubuntu 16.04-LTS (current default OS SKU) version of systemd is not using that version:

$ systemd --version
systemd 229
+PAM +AUDIT +SELINUX +IMA +APPARMOR +SMACK +SYSVINIT +UTMP +LIBCRYPTSETUP +GCRYPT +GNUTLS +ACL +XZ -LZ4 +SECCOMP +BLKID +ELFUTILS +KMOD -IDN

Here is the recently reverted PR:

#410

Issue Fixed:

Requirements:

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

Notes:

[jackfrancis]

@khenidak for functional review

@CecileRobertMichon who is working on 18.04-LTS integration: which version of systemd will Azure + 18.04-LTS deliver?

[CecileRobertMichon]

This configuration was recently removed

can we link that PR to this one?

which version of systemd will Azure + 18.04-LTS deliver?

I will find out.

[jackfrancis]

@CecileRobertMichon reference to #410 added in (this) PR description

[khenidak]

/lgtm

ps. thanks for accelerating this.

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[codecov]

Codecov Report

Merging #605 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #605      +/-   ##
==========================================
+ Coverage   56.68%   56.69%   +<.01%     
==========================================
  Files          91       91              
  Lines       13902    13905       +3     
==========================================
+ Hits         7881     7884       +3     
  Misses       5355     5355              
  Partials      666      666