This patch unifies /etc/motd managing approach for both RedHat and
Ubuntu systems. Providing a string value via linux:system:motd
pillar will configure static /etc/motd and remove dynamic scripts
from /etc/update-motd.d (if present).

update-motd can safely be removed because Ubuntu supports dynamic
motd by pam_motd means since 2009.

Related-Prod: PROD-17287

Change-Id: Ic9b7e18abb12cfe8704717b14dc1237e40715319

iteritems() was dropped in recent Python3 releases and items() is compatible
with Python 2.7.