pcs should talk with systemd via DBus to prevent unnecessary process spawning

Systemd-affiliated project supposedly contains all that's needed
so as to communicate with systemd:

https://github.com/systemd/python-systemd

It makes little sense from technical perspective to use anything
else (`systemctl` is a process-separated executable form to
trigger the same DBus calls, but since it is doable in-process
with said systemd binding, the former is just an inefficient version
of the latter for the purpose of pcs).

(Note that now that pacemaker high-level API is under development,
it will come into a similar situation, eventually, just instead of DBus
calls, there will have to be a direct Python binding built around.)