add namespace to nomad_job

[cgbaker]

some users have requested the ability to specify the namespace on a nomad job without having explicitly putting it into the job spec. this can be done by taking the existing computed attribute namespace and allowing it to be (optionally) specified.

specifically, would like to be able to do something like this:

provider "nomad" {
  address = "http://localhost:4646"
}

resource "nomad_namespace" "mynamespace" {
  name = "mynamespace"
}

resource "nomad_job" "myjob" {
  jobspec = file("${path.module}/myjob.nomad")
  detach = false
  namespace = nomad_namespace.mynamespace.name
}

[cgbaker]

because this is a computed field, making it also optional (so that users can specify it) causes a bunch of problems with the terraform plugin SDK, as it currently stands. we can revisit this later; they plan is to provide a new Nomad job resource which is structured to allow all viable fields at the top level.

[lgfa29]

Closing this for now as it has the same underlying issue of #1. We will address this problem in #149.