nomad-apm: add support to group nodes by datacenter

[jrasell]

When expanding a nomad-apm short form query, the nomad-autoscaler will check if the target has node_class or datacenter set to expand the query. When expanding the query though, node_class is assumed and therefore makes the use of datacenter with the nomad-apm plugin impossible. The way in which the node pool query is generated will need to be changed to utilise this helper function. These changes should be enough to enable support.

We should also consider supporting the combined node pooling functionality, however, this will require additional changes to the query expansion than described above. Whoever implements the initial datacenter support should make this choice, and raise follow up issues as needed.