Don't create a NetworkIndex when scheduling jobs that don't use networking

[schmichael]

Nomad's NetworkIndex struct serves 3 purposes during scheduling:

1. Assigning an IP address for each network.port stanza
2. Ensuring the job's static ports are not already in use on the node being ranked
3. Choosing an unused port for job's dynamic ports

However we build this index regardless of whether or not a job even requires networking! Not only does this impact performance, but it means any bugs in NetworkIndex (such as #13505) affect jobs which don't even require networking!

Sadly I don't think making computing the NetworkIndex conditional will be straightforward since where we initialize the NetworkIndex and where we check for the existence of network stanzas and use the NetworkIndex are spread apart.