(PUP-1932) Restrict systemd provider to manage file-based services

Prior to this commit, the systemd provider returned in-memory services.
This had a couple odd consequences, e.g. with 'puppet resource service':
1) short-lived services would come and go, though they are not actually
   managed directly
2) disabled and stopped services wouldn't be reported at all

This commit changes the systemd provider to return file-backed
services.
Merge pull request #2507 from kylog/issue/pup-1932/systemd-reports-transient-services

(PUP-1932) Restrict systemd provider to manage file-based services