We aim for containers to be part of a new cgroups hierarchy independent
from nomad agent.  However, we've been setting a relative path as
libcontainer `cfg.Cgroups.Path`, which makes libcontainer concatinate
the executor process cgroup with passed cgroup, as set in [1].

By setting an absolute path, we ensure that all cgroups subsystem
(including `name=systemd` get a dedicated one).  This matches behavior
in Nomad 0.8, and behavior of how Docker and OCI sets CgroupsPath[2]

Fixes #5736

[1] https://github.com/hashicorp/nomad/blob/d7edf9b2e42348865908735996359c7869fb16b5/vendor/github.com/opencontainers/runc/libcontainer/cgroups/fs/apply_raw.go#L326-L340
[2] https://github.com/moby/moby/blob/238f8eaa31aa74be843c81703fabf774863ec30c/vendor/github.com/containerd/containerd/oci/spec.go#L229