Use user provided image name to launch container

Support bind and tmpfs docker mounts

Co-Authored-By: notnoop <mahmood@notnoop.com>

Port metric prefix filtering to master

make: Fix invocation of vault e2e test

vault: protect against empty Vault secret response

Fixes #4921

Sadly, we don't have proper mechanism to mock Vault client, so not sure how to best test this.

I inspected the Vault client interactions, specially for cases where returned value is nil even if the error is also nil.  I believe we covered all correctly now:
* [`v.client.Sys().InitStatus()`](https://github.com/hashicorp/nomad/blob/f3853f11daa51336a2d46d883b1aff6feeddc7ec/nomad/vault.go#L427) - the value is non-nil boolean
* [`v.client.Sys().CapabilitiesSelf(path)`](https://github.com/hashicorp/nomad/blob/f3853f11daa51336a2d46d883b1aff6feeddc7ec/nomad/vault.go#L812): Capabilities handles empty bodies in [`hasCapability`](https://github.com/hashicorp/nomad/blob/f3853f11daa51336a2d46d883b1aff6feeddc7ec/vendor/github.com/hashicorp/vault/api/sys_capabilities.go#L43-L45) - also the `nil` array is handled with proper fail-safe default.
* [`v.client.Logical().Read(fmt.Sprintf("auth/token/roles/%s", role))`](https://github.com/hashicorp/nomad/blob/f3853f11daa51336a2d46d883b1aff6feeddc7ec/nomad/vault.go#L834-L840) handles when `rsecret` is nil

Also, fix a case where a successful second attempt of loading token can
cause a panic.

change to testing utilities to fix rkt tests

add generic plugin manager interface and orchestration

testing: in MkAllocDir, do not update TaskConfig with All() from the task builder, just with Env() (because it pollutes environment variables with node attributes and fails the rkt tests)

Modify fingerprint interface to use typed attribute struct

Device hook and devices affect computed node class

Add LXC driver

Looking at NewTaskRunner I'm unsure whether TaskRunner.TaskResources
(from which req.TaskResources is set) is intended to be nil at times or
if the TODO in NewTaskRunner is intended to ensure it is always non-nil.

The old approach was incomplete. Hook env vars are now:

 * persisted and restored between agent restarts
 * deterministic (LWW if 2 hooks set the same key)

This PR introduces a device hook that retrieves the device mount
information for an allocation. It also updates the computed node class
computation to take into account devices.

TODO Fix the task runner unit test. The environment variable is being
lost even though it is being properly set in the prestart hook.

Fix deadlock in previous alloc watcher by emitting last alloc update

drivers/rkt: fix invalid volumes

taskrunner: Return the restart delay correctly