Emit a specification file for each fixture.

[sholsapp]

We need to emit a specification file for a fixture that lends itself to being read programatically. This spec file would describe the fixture itself: how many nodes, what are their short names, what are the IP addresses, what are their port mappings, what are the start and stop commands, etc.

This spec file can then be read and used within an integration test and helps test authors avoid hard coding fixture-specific logic into their unit tests.