Detect and support both systemd and sysvinit on Fedora, RHEL and CentOS

Detect and support both systemd and sysvinit on Fedora, RHEL and CentOS

To properly install systemd-enabled and old (sysvinit) Ceph packages
we need to properly detect when systemd target is used.

This cannot be done just by checking distro version, it depends
on Ceph package content (sysvinit can be used on systemd distro as well).

Let's check ceph.target existence and enable systemd only in this case.

Signed-off-by: Milan Broz <mbroz@redhat.com>