E2E: advertise serf on secondary IP

[tgross]

While testing #16490 and #16217 I added a secondary network to our test infrastructure, but the Nomad configuration in our nightly tests doesn't use it by default. The secondary network doesn't get configured automatically by cloud-init either, so there's a little bit of infra work to do here on the machine image.

• Update cloud-init in our packer build to do the equivalent of the following, based on the IP addresses we :

# find the IP address, there's probably a cloud-init native way to do this
mac=$(curl -s 'http://169.254.169.254/latest/meta-data/network/interfaces/macs/' | tail -1)
curl -s "http://169.254.169.254/latest/meta-data/network/interfaces/macs/$mac/local-ipv4s"

# set that address for the 2nd device
ip link set dev ens6 up
ip addr add 172.31.x.y dev ens6
ip route add 172.31.x.0/20 via 172.31.x.y

Edit: there seems to be something flaky where sometimes the NIC will get configured but not always. Probably a race during startup and attaching the NIC. That'll be something to debug or assert here.

• Generate the following config file from the appropriate IP address, and add it to the files we upload to the servers:

addresses {
  serf = "{{ GetPrivateInterfaces | include \"network\" \"172.31.x.0/20\" | limit 1 | attr \"address\" }}"
}

advertise {
  serf = "{{ GetPrivateInterfaces | include \"network\" \"172.31.x.0/20\" | limit 1 | attr \"address\" }}"
}