adding support for systemd env options

[aleiner]

What type of PR is this?

(REQUIRED)

• bug
• cleanup
• documentation
• feature

What this PR does / why we need it:

It adds the optional ability to add and configure an rke-{server,agent} systemd environment file to /etc/default

Which issue(s) this PR fixes:

Fixes #169
Rework of #174

Release Notes

NONE

[Falltrades]

Hi @mddamato, @aleiner,
I tested the deployment on my side and it seems everything is working properly. Is there anything else I can help with this PR ?

[Falltrades]

Hi @aleiner,
I wonder if the team might be interested in a PR to fix Lint warnings ? (At least for those we are seeing in this PR)

[aleiner]

Hi @aleiner, I wonder if the team might be interested in a PR to fix Lint warnings ? (At least for those we are seeing in this PR)

Thanks for the offer and sorry for all of the delays. We have something in the works that we're hoping to push out soon.