Update configuring-tasks.html.md

Update ecs.html.md

website: add ‘intro to nomad’ video to /intro

Return FailedTGAlloc metric instead of no node err

If an existing system allocation is running and the node its running on
is marked as ineligible, subsequent plan/applys return an RPC error
instead of a more helpful plan result.

This change logs the error, and appends a failedTGAlloc for the
placement.

Fixes a bug introduced in 0aa58b90 where we're writing a test file to
a taskdir-interpolated location, which works when we `alloc exec` but
not in the jobspec for a group script check.

This changeset also makes the test safe to run multiple times by
namespacing the file with the alloc ID, which has the added bonus of
exercising our alloc interpolation code for group script checks.

The e2e framework instantiates clients for Nomad/Consul but the
provisioning of the actual Nomad cluster is left to Terraform. The
Terraform provisioning process uses `remote-exec` to deploy specific
versions of Nomad so that we don't have to bake an AMI every time we
want to test a new version. But Terraform treats the resulting
instances as immutable, so we can't use the same tooling to update the
version of Nomad in-place. This is a prerequisite for upgrade testing.

This changeset extends the e2e framework to provide the option of
deploying Nomad (and, in the future, Consul/Vault) with specific
versions to running infrastructure. This initial implementation is
focused on deploying to a single cluster via `ssh` (because that's our
current need), but provides interfaces to hook the test run at the
start of the run, the start of each suite, or the start of a given
test case.

Terraform work includes:
* provides Terraform output that written to JSON used by the framework
  to configure provisioning via `terraform output provisioning`.
* provides Terraform output that can be used by test operators to
  configure their shell via `$(terraform output environment)`
* drops `remote-exec` provisioning steps from Terraform
* makes changes to the deployment scripts to ensure they can be run
  multiple times w/ different versions against the same host.

I originally planned to add component documentation, but as this dragged on and I found that JSDoc-to-Markdown sometimes needed hand-tuning, I decided to skip it and focus on replicating what was already present in Freestyle. Adding documentation is a finite task that can be revisited in the future.

My goal was to migrate everything from Freestyle with as few changes as possible. Some adaptations that I found necessary:
• the DelayedArray and DelayedTruth utilities that delay component rendering until slightly after initial render because without them:
  ◦ charts were rendering with zero width
  ◦ the JSON viewer was rendering with empty content
• Storybook in Ember renders components in a routerless/controllerless context by default, so some component stories needed changes:
  ◦ table pagination/sorting stories access to query params, which necessitates some reaching into Ember internals to start routing and dynamically generate a Storybook route/control...

Group service checks cannot interpolate task fields, because the task
fields are not available at the time the script check hook is created
for the group service. When f31482ae was merged this e2e test began
failing because we are now correctly matching the script check ID to
the service ID, which revealed this jobspec was invalid.

This builds on API changes in #6017 and #6021 to conditionally turn off the
“Run Job” button based on the current token’s capabilities, or the capabilities
of the anonymous policy if no token is present.

If you try to visit the job-run route directly, it redirects to the job list.