tests: Migrated allocs aren't lost

Fix `TestServiceSched_NodeDown` for checking that the migrated allocs
are actually marked to be stopped.

The boolean logic in test made it skip actually checking client status
as long as desired status was stop.

Here, we mark some jobs for migration while leaving others as running,
and we check that lost flag is only set for non-migrated allocs.

Fix a website link under `Runtime Environment`

Under the `Network-related Variables` the `NOMAD_HOST_PORT_<label>` has
a incorrect link.

Proposing new tfvars with additional inline docs

More test fixes

set node.StatusUpdatedAt in raft

Fix restart attempts of `restart` stanza in `delay` mode.

Revert 0.9.2 release super script tags

nomad: disable service+batch preemption by default

Add Enterprise docs for Preemption

Enterprise only.

Disable preemption for service and batch jobs by default.

Maintain backward compatibility in a x.y.Z release. Consider switching
the default for new clusters in the future.

client config flag to disable remote exec

nomad: revert use of SnapshotAfter in planApply

Cleanup docker images

Revert plan_apply.go changes from #5411

Since non-Command Raft messages do not update the StateStore index,
SnapshotAfter may unnecessarily block and needlessly fail in idle
clusters where the last Raft message is a non-Command message.

This is trivially reproducible with the dev agent and a job that has 2
tasks, 1 of which fails.

The correct logic would be to SnapshotAfter the previous plan's index to
ensure consistency. New clusters or newly elected leaders will not have
a previous plan, so the index the leader was elected should be used
instead.

This exposes a client flag to disable nomad remote exec support in
environments where access to tasks ought to be restricted.

I used `disable_remote_exec` client flag that defaults to allowing
remote exec. Opted for a client config that can be used to disable
remote exec globally, or to a subset of the cluster if necessary.

- modified tests to cleanup now that RemoveContainer isn't in StartTask
- fix some broken tests by removing docker images/containers before test

docker: DestroyTask was not cleaning up Docker images because it was erroring early due to an attempt to inspect an image that had already been removed

drivers/exec: Restore 0.8 capabilities