Fixes #2478 #2474 #1995 #2294

The new client only handles agent and task service advertisement. Server
discovery is mostly unchanged.

The Nomad client agent now handles all Consul operations instead of the
executor handling task related operations. When upgrading from an
earlier version of Nomad existing executors will be told to deregister
from Consul so that the Nomad agent can re-register the task's services
and checks.

Drivers - other than qemu - now support an Exec method for executing
abritrary commands in a task's environment. This is used to implement
script checks.

Interfaces are used extensively to avoid interacting with Consul in
tests that don't assert any Consul related behavior.

Fix variable capture and add tests

This PR fixes token revocation and adds tests to make sure it is
working. The 0.5.6 RC1's token revocation does not work becasue the
token's value is captured at the instantiation of the deferred
stoprenewal statement rather than its exectution.

Fix typo and be consistent in naming

Vendor consul & consul-template

Improve artifact download error message

Don't take a reference a var in a loop

disk is not a supported entry at task level

Removed the reference to `disk` in the `resources` overview statement

Stop Vault token renew on task exit

Fix dispatch of periodic job

Various fixes for setting user/group of task

Access Node Meta and Attrs in template

Fix periodic job state

Proper reference counting through task restarts

This PR fixes an oversight in which the client would attempt to renew a
token even after the task exits.

Fixes https://github.com/hashicorp/nomad/issues/2475