Applying technique from #773 to is_enabled property

[pfuntner]

This applies the same technique from #773 to the is_running property of the SystemdService class of testinfra/modules/service.py to fix #801.

I tried following the steps from CONTRIBUTING.rst but got some errors from tox while installing timelib: tox-output.txt.

• Are there prerequisites I'm missing? If so, can CONTRIBUTING.rst be improved?
• The automated tests were successful. Are they sufficient?

Let me know if I can do anything to get this merged.

[pfuntner]

@philpep Is there anything I can do to move this along? It's a simple change.