executor: stop joining executor to container cgroup
executor: stop joining executor to container cgroup

Stop joining libcontainer executor process into the newly created task
container cgroup, to ensure that the cgroups are fully destroyed on
shutdown, and to make it consistent with other plugin processes.

Previously, executor process is added to the container cgroup so the
executor process resources get aggregated along with user processes in
our metric aggregation.

However, adding executor process to container cgroup adds some
complications with much benefits:

First, it complicates cleanup.  We must ensure that the executor is
removed from container cgroup on shutdown.  Though, we had a bug where
we missed removing it from the systemd cgroup.  Because executor uses
`containerState.CgroupPaths` on launch, which includes systemd, but
`cgroups.GetAllSubsystems` which doesn't.

Second, it may have advese side-effects.  When a user process is cpu
bound or uses too much memory, executor should remain functioning
without risk of being killed (by OOM killer) or throttled.

Third, it is inconsistent with other drivers and plugins.  Logmon and
DockerLogger processes aren't in the task cgroups.  Neither are
containerd processes, though it is equivalent to executor in
responsibility.

Fourth, in my experience when executor process moves cgroup while it's
running, the cgroup aggregation is odd.  The cgroup
`memory.usage_in_bytes` doesn't seem to capture the full memory usage of
the executor process and becomes a red-harring when investigating memory
issues.

For all the reasons above, I opted to have executor remain in nomad
agent cgroup and we can revisit this when we have a better story for
plugin process cgroup management.

hashicorp/nomad,dvusboy/nomad,hashicorp/nomad,dvusboy/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad,hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad