Add checkmode support for test-module script

Signed-off-by: Hector Acosta <hector.acosta@gmail.com>
Merge pull request #8214 from hacosta/checkmode

Add checkmode support for test-module script
Revert "Explicitly set the user option for ssh connections"

This reverts commit 0f91add86f7d30e6d895e01bbfddb93a3b5e48df.

Fixes #8216
service: test properly whether systemd is running

Use a pure Python reimplementation of the [sd_booted(3)][1] function
from [systemd v210][2].

[1]: http://www.freedesktop.org/software/systemd/man/sd_booted.html
[2]: http://cgit.freedesktop.org/systemd/systemd-stable/tree/src/libsystemd/sd-daemon/sd-daemon.c?id=v210#n465