Needs to be more like http.

The parse endpoint accepts a hcl jobspec body within a json object
and returns the parsed json object for the job. This allows users to
register jobs with the nomad json api without specifically needing
a nomad binary to parse their hcl encoded jobspec file.

Fixes #3380

Adds address_mode to checks (but no auto) and allows services and checks
to set literal port numbers when using address_mode=driver.

This allows SDNs, overlays, etc to advertise internal and host addresses
as well as do checks against either.

add conversion for KillSignal for api/struct representation of task

Treat warnings as unhealthy by default

Fixes #2441

Defaults to 0 (no delay) for backward compat and because this feature
should be opt-in.

This PR allows a template to specify the Vault grace duration.

Fixes https://github.com/hashicorp/nomad/issues/2922

This PR:
* disallows update stanzas on batch jobs
* undeprecates the stagger field
* changes the way warnings are returned

Fixes #2678

Ideally DriverNetwork would be fully populated in Driver.Prestart, but
Docker doesn't assign the container's IP until you start the container.

However, it's important to setup the port env vars before calling
Driver.Start, so Prestart should populate that.