* client: ensure minimal cgroup controllers enabled
This PR fixes a bug where Nomad could not operate properly on operating
systems that set the root cgroup.subtree_control to a set of controllers that
do not include the minimal set of controllers needed by Nomad.
Nomad needs these controllers enabled to operate:
- cpuset
- cpu
- io
- memory
- pids
Now, Nomad will ensure these controllers are enabled during Client initialization,
adding them to cgroup.subtree_control as necessary. This should be particularly
helpful on the RHEL/CentOS/Fedora family of system. Ubuntu systems should be
unaffected as they enable all controllers by default.
Fixes: https://github.com/hashicorp/nomad/issues/14494
* docs: cleanup doc string
* client: cleanup controller writes, enhance log messages
Seth Hoenig