Add structs and fields to support the Nomad Pools Governance Enterprise
feature of controlling node pool access via namespaces.
Nomad Enterprise allows users to specify a default node pool to be used
by jobs that don't specify one. In order to accomplish this, it's
necessary to distinguish between a job that explicitly uses the
`default` node pool and one that did not specify any.
If the `default` node pool is set during job canonicalization it's
impossible to do this, so this commit allows a job to have an empty node
pool value during registration but sets to `default` at the admission
controller mutator.
In order to guarantee state consistency the state store validates that
the job node pool is set and exists before inserting it.
Implementation of the base work for the new node pools feature. It includes a new `NodePool` struct and its corresponding state store table.
Upon start the state store is populated with two built-in node pools that cannot be modified nor deleted:
* `all` is a node pool that always includes all nodes in the cluster.
* `default` is the node pool where nodes that don't specify a node pool in their configuration are placed.
