systemd services: fix installing ceph-volume@

systemd services: fix installing ceph-volume@

The ceph-volume@ service uses the @CMAKE_INSTALL_PREFIX@, so we cannot
just install it directly like the target units, but need to process it
like all other units that are referring to actual executables that
reside in build-target dependent paths.

Fixes: 68c72c5dff5 ("systemd: use CMake install prefix in templates")
Reported-by: Daniel Herzig <d.herzig@proxmox.com>
Reported-by: Aaron Lauterer <a.lauterer@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>