Retry if we detect shutting down during Start() api call is started,
locally.

driver/docker: collect tty container logs

Documentation for `nomad alloc stop`

Fixes https://github.com/hashicorp/nomad/issues/5475

When container is a tty container, we need to get raw terminal output
without any additional processing.

Normalize plan to increase the plan apply throughput

singularity_path -> singularity_cache

UI: Preemptions

Signed-off-by: Eduardo Arango <eduardo@sylabs.io>

Docs ea update 0.9

docs: add download link to 0.9.1-rc1

website: add plugin docs

fix crash when executor parent nomad process dies

Fixes https://github.com/hashicorp/nomad/issues/5593

Executor seems to die unexpectedly after nomad agent dies or is
restarted.  The crash seems to occur at the first log message after
the nomad agent dies.

To ease debugging we forward executor log messages to executor.log as
well as to Stderr.  `go-plugin` sets up plugins with Stderr pointing to
a pipe being read by plugin client, the nomad agent in our case[1].
When the nomad agent dies, the pipe is closed, and any subsequent
executor logs fail with ErrClosedPipe and SIGPIPE signal.  SIGPIPE
results into executor process dying.

I considered adding a handler to ignore SIGPIPE, but hc-log library
currently panics when logging write operation fails[2]

This we opt to revert to v0.8 behavior of exclusively writing logs to
executor.log, while we investigate alternative options.

[1] https://github.com/hashicorp/nomad/blob/v0.9.0/vendor/github.com/hashicorp/go-plugin/client.go#L528-L535
[2] https://github.com/hashicorp/nomad/blob/v0.9.0/vendor/github.com/hashicorp/go-hclog/int.go#L320-L323

alloc-lifecycle: nomad alloc stop

This adds a `nomad alloc stop` command that can be used to stop and
force migrate an allocation to a different node.

This is built on top of the AllocUpdateDesiredTransitionRequest and
explicitly limits the scope of access to that transition to expose it
under the alloc-lifecycle ACL.

The API returns the follow up eval that can be used as part of
monitoring in the CLI or parsed and used in an external tool.