Currently, nomad "plugin" processes (e.g. executor, logmon, docker_logger) are started as CLI
commands to be handled by command CLI framework.  Plugin launchers use
`discover.NomadBinary()` to identify the binary and start it.

This has few downsides: The trivial one is that when running tests, one
must re-compile the nomad binary as the tests need to invoke the nomad
executable to start plugin.  This is frequently overlooked, resulting in
puzzlement.

The more significant issue with `executor` in particular is in relation
to external driver:

* Plugin must identify the path of invoking nomad binary, which is not
trivial; `discvoer.NomadBinary()` now returns the path to the plugin
rather than to nomad, preventing external drivers from launching
executors.

* The external driver may get a different version of executor than it
expects (specially if we make a binary incompatible change in future).

This commit addresses both downside by having the plugin invocation
handli...

Fix incorrect link to the sample Grafana dashboard

consul: Include port-label in service registration

Updated minimum development go version

It is possible to provide multiple identically named services with
different port assignments in a Nomad configuration.

We introduced a regression when migrating to stable service identifiers where
multiple services with the same name would conflict, and the last definition
would take precedence.

This commit includes the port label in the stable service identifier to
allow the previous behaviour where this was supported, for example
providing:

```hcl
service {
  name = "redis-cache"
  tags = ["global", "cache"]
  port = "db"
  check {
    name     = "alive"
    type     = "tcp"
    interval = "10s"
    timeout  = "2s"
  }
}

service {
  name = "redis-cache"
  tags = ["global", "foo"]
  port = "foo"

  check {
    name     = "alive"
    type     = "tcp"
    port     = "db"
    interval = "10s"
    timeout  = "2s"
  }
}

service {
  name = "redis-cache"
  tags = ["global", "bar"]
  port = "bar"

  check {
    name     = "alive"
    type     = "tcp"
    port     = "db"
    interval = "10s"
    timeout  = "2s"
  }
}
```

in a nomad task definition is now completely valid. Each service
definition with the same name must still have a unique port label however.

Optional Consul service tags for nomad server and agent services

Add warning docs around downgrades

Assorted minor changes

trhooks: Add TaskStopHook interface to services

documentation: fixed incorrect URLs in allocation signal/restart documentation

alloc-lifecycle: Fix restart with empty body

docs: Add missing docs for alloc restart api

We currently only run cleanup Service Hooks when a task is either
Killed, or Exited. However, due to the implementation of a task runner,
tasks are only Exited if they every correctly started running, which is
not true when you recieve an error early in the task start flow, such as
not being able to pull secrets from Vault.

This updates the service hook to also call consul deregistration
routines during a task Stop lifecycle event, to ensure that any
registered checks and services are cleared in such cases.

fixes #5770

Currently when you submit a manual request to the alloc lifecycle API
with a version of Curl that will submit empty bodies, the alloc restart
api will fail with an EOF error.

This behaviour is undesired, as it is reasonable to not submit a body at
all when restarting an entire allocation rather than an individual task.

This fixes it by ignoring EOF (not unexpected EOF) errors and treating
them as entire task restarts.

fix consul stanza parsing when multiple -config arguments are used

This reverts commit 006a9a1d.

fix consul stanza parsing when a configuration directory is used