Add tests to base image builds: Validate image contents with serverspec

What does this MR do?

⚠ commits are individually reviewable.

Uses the Serverspec ruby gem to validate image contents. At this stage, only the base role is tested. Further MRs (!90 (merged)) will add specs for other roles.

Why was this MR needed?

We need to check that the images we're release are actually working, in an automated fashion

What's the best way to test this MR?

scripts/dev-vm --base-image base-11.img new
scripts/dev-vm --base-image base-11.img --role base spec

# specs should pass!

What are the relevant issue numbers?

See #39 (closed)