chore(rootfs/Dockerfile): update Azure CLI to 2.3.1

[CecileRobertMichon]

This requires using pip3. Installing az-cli 2.3.x fails with

Collecting mock~=4.0 (from azure-cli)
  Could not find a version that satisfies the requirement mock~=4.0 (from azure-cli) (from versions: 0.5.0, 0.6.0, 0.7.0b1, 0.7.0b2, 0.7.0b3, 0.7.0b4, 0.7.0rc1, 0.7.0, 0.7.1, 0.7.2, 0.8.0, 1.0b1, 1.0.0, 1.0.1, 1.1.0, 1.1.1, 1.1.2, 1.1.3, 1.1.4, 1.2.0, 1.3.0, 2.0.0, 3.0.0, 3.0.1, 3.0.2, 3.0.3, 3.0.4, 3.0.5)
No matching distribution found for mock~=4.0 (from azure-cli)

According to https://docs.microsoft.com/en-us/cli/azure/install-azure-cli-linux?view=azure-cli-latest#prerequisites, "The CLI has dropped support for Python 2.7 since version 2.1.0. New versions no longer guarantee to run with Python 2.7 correctly."

[mboersma]

Python is here really just to install az, so I think we can switch over entirely from 2 to 3. Let me try locally and then I'll push a commit if that works.