staging: Dedicate an indexer worker

The end goal is to replicate our current production environment. And making sure writing
to the journal with more than 1 indexer behave properly.

terraform apply summary:

worker3_summary =
hostname: worker3
fqdn: worker3.internal.staging.swh.network
network: ip=192.168.130.103/24,gw=192.168.130.1 macaddrs=1A:F8:1A:2C:12:E1

Related to T2780