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

[jnpkrn]

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.)

[tomjelinek]

I took a look at the library. It seems to me it doesn't provide means for starting and stopping daemons and querying their status currently, which is what pcs does the most regarding systemd. We will stick to running systemctl for now.