docs: task driver resource isolation & security

In Nomad prior to Consul Connect, all Consul checks work the same
except for Script checks. Because the Task being checked is running in
its own container namespaces, the check is executed by Nomad in the
Task's context. If the Script check passes, Nomad uses the TTL check
feature of Consul to update the check status. This means in order to
run a Script check, we need to know what Task to execute it in.

To support Consul Connect, we need Group Services, and these need to
be registered in Consul along with their checks. We could push the
Service down into the Task, but this doesn't work if someone wants to
associate a service with a task's ports, but do script checks in
another task in the allocation.

Because Nomad is handling the Script check and not Consul anyways,
this moves the script check handling into the task runner so that the
task runner can own the script check's configuration and
lifecycle. This will allow us to pass the gr...

driver/docker: convert host bind path to os native

When summing up the capability of your Nomad fleet for scaling purposes, it's important to exclude draining nodes, as they won't accept new jobs.

* connect: add unix socket to proxy grpc for envoy

Fixes #6124

Implement a L4 proxy from a unix socket inside a network namespace to
Consul's gRPC endpoint on the host. This allows Envoy to connect to
Consul's xDS configuration API.

* connect: pointer receiver on structs with mutexes

* connect: warn on all proxy errors

docker: Fix driver spec

hclspec.NewLiteral does not quote its values, which caused `3m` to be
parsed as a nonsensical literal which broke the plugin loader during
initialization. By quoting the value here, it starts correctly.

Add default value "3m" to image_delay, making it consistent with docs.

Remove unused canary param from MakeTaskServiceID

docs: Update /v1/node/{node-id} example response

bumped hadoop version

Update AWS logo to current version

e2e: filter default AMI by OS

ci: Migrate remaining jobs to CircleCI

rkt deprecation notice

Add an OS tag to Packer builds of our e2e test AMIs and then filters
by this in Terraform.

consul: ignore connect services when syncing

Consul registers Connect services automatically, however Nomad thinks it
owns them due to the _nomad prefix. Since the services are managed by
Consul, Nomad needs to explicitly ignore them or otherwies they will be
removed.

chore: Remove Go Tests from Travis

raw_exec: be defensive when disabled

This fixes the ExecTask definition to match with the DriverPlugin
interface.

schedulers: check all drivers on node