network: expose network namespace configured IP to task env

[jrasell]

When utilising the Nomad network stanza and specifically mode = bridge, Nomad will create a network namespace and have CNI configure the namespace with a private IP address according to the configuration within networking_bridge_linux. This IP is the only address, alongside the loopback which is configured in the network namespace.

Applications that cluster together, such as Vault utilising Raft storage require exposing an IP that other members can connect to for RPC. Due to its nature, this IP must be routable from outside the container.

Nomad does not expose the configured IP to the task and therefore it is up to the task to figure out what IP address it must configure for binding and exposure. This is possible if the application utilises go-sockaddr, but this is a specific requirement that not even Vault can realise currently (Vault pr hashicorp/vault#9109).

Nomad should be enhanced so that any configured CNI address is exposed to the task as an environment variable in the same manner that NOMAD_HOST_, NOMAD_ADDR_, and NOMAD_IP_ variables are exposed. This would then allow tasks to read this variable in using Nomad interpolation or template stanzas.