staging: Bootstrap infrastructure with the gateway node

This provisions the node and for the first time, will trigger a puppet run.

This puppet run will install the gateway's role including the routes back/forth between production network and the staging network.

Note:

In the provision steps, prior to call puppet, we do need to install some setup ourselves (route, ip_forward, etc...) to allow proxmox to continue the provisioning of other nodes (coming). Puppet run installs the equivalent instruction to make them persistent.

Related T1785 Related T1874

Depends on !29 (closed)

Test Plan

terraform init
terraform plan

This demos what it wants to do

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Migrated from D1797 (view on Phabricator)