Disable Ubuntu motd banner and provide a Delphix-specific one.

[sebroy]

ab-pre-push: http://selfservice.jenkins.delphix.com/job/devops-gate/job/master/job/appliance-build-orchestrator-pre-push/2577/

Before:

guild login: sysadmin
Password:
Welcome to Ubuntu 18.04.3 LTS (GNU/Linux 5.0.0-36-generic x86_64)
 * Documentation:  https://help.ubuntu.com
 * Management:     https://landscape.canonical.com
 * Support:        https://ubuntu.com/advantage
 * Overheard at KubeCon: "microk8s.status just blew my mind".
     https://microk8s.io/docs/commands#microk8s.status
 * Canonical Livepatch is available for installation.
   - Reduce system reboots and improve kernel security. Activate at:
     https://ubuntu.com/livepatch
The programs included with the Ubuntu system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.
Ubuntu comes with ABSOLUTELY NO WARRANTY, to the extent permitted by
applicable law.
guild.dc1>

After:

guild login: sysadmin
Password:
Last login: Mon Dec  9 16:01:40 UTC 2019 on ttyS0
Welcome to Delphix 6.1.0.0-snapshot.20191127042627846
guild.dc1>

[sebroy]

Update on this PR: During testing, I noticed that the motd-news.service was in failed state. In its journal was:

Dec 10 19:27:08 guild.dc1 systemd[1]: Starting Message of the Day...
Dec 10 19:27:08 guild.dc1 systemd[13914]: motd-news.service: Failed to execute command: Permission denied
Dec 10 19:27:08 guild.dc1 systemd[13914]: motd-news.service: Failed at step EXEC spawning /etc/update-motd.d/50-motd-new

This lead me down the rabbit hole of motd-news.service, which fetches dynamic content from the Internet every 12 hours via a motd-news.timer systemd unit. I'm going to be updating this PR to disable this, as not only does this fail if motd scripts are disabled, but we don't want engines reaching out to the public Internet for dynamic content from Ubuntu.