Port systemd gatherer

[arbulu89]

Port systemd gatherer to use the new FactValue object. The used argument is the name of the daemon to get the state.
It returns an active/inactive string. If the deamon is disabled or does not even exist, inactive is returned.
We could actually return more things as map if needed. All of this
Ref.

facts:
  - name: sbd_state
    gatherer: systemd
    argument: sbd

PD:
I have renamed some constants from other gatherers to use more meaningful names.

[rtorrero]

LGTM and it actually reminded me that we had agreed on changing the FactKey naming for something more understandable and that I need to change the testing package name with the _test in my last PRs too 😅

[arbulu89]

LGTM and it actually reminded me that we had agreed on changing the FactKey naming for something more understandable and that I need to change the testing package name with the _test in my last PRs too sweat_smile

Aaa yes!
We should start looking on that in new pieces of code.
I forgot in the review...