In a few places Nomad was using flag implementations directly
from Consul, lending to Nomad's need to import consul. Replace
those uses with helpers already in Nomad, and copy over the bare
minimum needed to make the autopilot flags behave as they have.
The call to render the output diff swapped the `diff` and `verbose` bool
parameters, resulting in dropping the diff output in multi-region plans but
not single-region plans.
Add a scatter-gather for multiregion job plans. Each region's servers
interpolate the plan locally in `Job.Plan` but don't distribute the plan as
done in `Job.Run`.
Note that it's not possible to return a usable modify index from a multiregion
plan for use with `-check-index`. Even if we were to force the modify index to
be the same at the start of `Job.Run` the index immediately drifts during each
region's deployments, depending on events local to each region. So we omit
this section of a multiregion plan.
