Set SELinux enforcing mode

[frelon]

SELinux enforcing mode was removed from green flavor in 1114b12 while trying to get all tests working.

The goal is to reenable SELinux enforcing mode for green flavor.

[frelon]

Pausing this for now. Some key takeaways from debugging this issue:

• The error encountered is Non unix-domain-socket passed as listener, in the dbus-broker service during boot.
• When starting a service that depends on dbus-broker after logging into the machine, dbus-broker is started without any error.
• The same error is found when trying to boot ubuntu 21.10 with selinux tools installed and booting in enforcing mode.
• It works when booting into systemd-rescue first and then exiting and booting into the active system.