Cleanup Systemd files and add Travis-CI test for Systemd

[jamrok]

Observation:
I was testing with Molecule on an Ubuntu 18.04 host and each time I created privileged containers using your Debian based images that support Systemd, it logged me out (Red Hat based images worked without a problem). Creating the containers manually caused the same problem (as expected).

Root Cause:
Extraneous Systemd files + privileged mode = troubling times in the kingdom.

Recommendation:

• Following the info here https://hub.docker.com/_/centos/, we should clean up those unwanted Systemd files on Debian based systems as well. The main difference with my change is that I used find to do the same cleanup as listed here.
• The change in this PR allows Systemd to run properly inside Ubuntu and Debian based containers without affecting the host (helps for local testing).
• This change should be applied to your Ubuntu 16.04, Debian 8 and Debian 9 repositories as well.

Testing:
The following shows the Systemd status when the files are present (your image before the change):
https://travis-ci.com/jamrok/docker-ubuntu1804-ansible/builds/101129316#L1102-L1135

The following shows the Systemd status when the files are removed (after the change):
https://travis-ci.com/jamrok/docker-ubuntu1804-ansible/jobs/178182250#L1106-L1120

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I was checking your Issues and it seems this might solve for #9 as well.

Let me know what you think,

Thanks

[geerlingguy]

Can you clean up the changes based on #10?