add new pve-container units

add new pve-container units

to replace lxc@.service.

we want our unit to only start when manually invoked (by our code), and
stop on shutdown via pve-guests or pve-ha-lrm. lxc@ units are stopped by
systemd on shutdown, because of transitive dependencies.

since all instances of template service units are by default assigned to
a new slice with DefaultDependencies=yes, we also need to introduce our
own custom slice with DefaultDependencies=no.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>