Fix HCL2 panics when parsing undefined variables

* website: add customer testimonial to homepage

* website: remove one testimonial, remove product logos

makefile cleanup

With the updated undefined variable code, we attempt to pick the text of
`${....}` verbatim from the hcl body. Previously, we'd attempt to
regenerate the string from the AST and pray it matches input; the
generation is lossy, as the same AST can represent multiple variations
(e.g. `${v.0}` and `${v[0]}` have the same HCLv2 AST). In this change,
we attempt to go back to the hcl2 source and find the string snippet
corresponding to the variable reference.

Update hcl/v2 to point to https://github.com/hashicorp/hcl/commits/nomad-v2.9.1+tweaks1

Updates stackmenu to latest

Our API client `delete` method doesn't include a request body, but accepts an
interface for the response. We were accidentally putting the request body into
the response, which doesn't get picked up in unit tests because we're not
reading the (always empty) response body anyways.

Consumers of the CSI HTTP API are expecting a response object and not a slice
of snapshots. Fix the return value.

The plugin stub object does not include fine-grained capability checks, which
means `nomad volume status -verbose` will return ugly and verbose error
"Unimplemented" messages from the plugin if it does not support the CSI
`ListVolumes` RPC. Return a nicer error message from our RPC handler instead.

The HTTP router did not correctly route `/v1/volumes/external` without being
explicitly added to the top-level router. Break this out into its own request
handler.

If a plugin returns an error, we should continue at the outer scope to query
the next plugin, otherwise we just retry the plugin we got an error
on (potentially infinitely if it's an invalid request like an unsupported
plugin).

Add new `access_mode`/`attachment_mode` fields. Make it more clear which set
of fields belong to create vs register. Update the example spec that's
generated by `volume init`.

Registration of Nomad volumes previously allowed for a single volume
capability (access mode + attachment mode pair). The recent `volume create`
command requires that we pass a list of requested capabilities, but the
existing workflow for claiming volumes and attaching them on the client
assumed that the volume's single capability was correct and unchanging.

Add `AccessMode` and `AttachmentMode` to `CSIVolumeClaim`, use these fields to
set the initial claim value, and add backwards compatibility logic to handle
the existing volumes that already have claims without these fields.

fix potential race condition in API client

enable -race detector for testing api

build: install buf during bootstrap

Previously installing buf was left out of `make bootstrap` because it
had conflicts with the `tools/go.mod` file and dependencies used by
other tools. With Go 1.16 we eliminated that `go.mod` file, and can
now just install `buf` with `go install` like everything else.

Fix the broken link in the bootstrapping custom scheduler warning

consul: plumbing for specifying consul namespace in job/group

Co-authored-by: Tim Gross <tgross@hashicorp.com>

This PR adds the common OSS changes for adding support for Consul Namespaces,
which is going to be a Nomad Enterprise feature. There is no new functionality
provided by this changeset and hopefully no new bugs.