Use Arcus for CI

[sjpb]

Add CI on Arcus. Unlike the existing smslabs CI, this uses direct-type VNICs so can test RDMA (using "inbox" drivers).

To avoid duplication between smslabs and arcus environments, this PR:

• Replaces the (outdated, unused, unloved) terraform in the cookiecutter skeleton with terraform based on smslabs, but modified to optionally support direct-mode ports VNICs
• Uses that terraform as a module from both arcus and smslabs environments
• Uses the everything layout as the arcus and smslabs environment's groups.
• Uses a single workflow file with matrix to select which cloud.

NB: the same image (see environments/*/terraform/main.tf) is used for both arcus and smslabs.

Arcus also exposed a bug in hpctests pingmatrix output (divide-by-zero when no difference between min and max latency), fixed here in baea143 & 02f9475 so that CI passes.

TODO:

• check packer build base