Carthagenet fix + modularization

[nicolasochem]

This lets you provide a carthagenet snapshot link. Example that worked for me right now:

tezos_network="carthagenet"
baking_nodes = {
  mynode = {
    mybaker = {
      public_baking_key="tz1YmsrYxQFJo5nGj4MEaXMPdLrcRf2a5mAU"
      insecure_private_baking_key="edsk3cftTNcJnxb7ehCxYeCaKPT7mjycdMxgFisLixrQ9bZuTG2yZK"
    }
  }
}
full_snapshot_url = "https://snaps.tulip.tools/carthagenet_2020-08-09_20:00.full"
rolling_snapshot_url = "https://snaps.tulip.tools/carthagenet_2020-08-09_20:00.rolling"
org_id = "changeme"
billing_account = "changeme"
terraform_service_account_credentials="changeme"

Other changes:

• more modular spec for baking nodes. Lets you specify several baking addresses, connected to several remote signers, and instantiate several private nodes.
• prometheus is deployed in the cluster
• deploys in its own namespace
• support for more regions than us-central1 (will alter the regional PV format)