consolidate resources and simplify provider usage

[joshrwolf]

Remove the confusing primitives (environment and test) and replace it with the simplified:

• feature
• harness*

Now instead of needing to plumb tests through environment -(test)-> feature, you simply leverage the terraform apply as the environment manager and define feature resources.

This also changes the default behavior of the tests to happen in isolated environments (containers), which are configured through the use of harnesses.

The example provided in examples/resources/main.tf is an updated example of the new workflow.

[joshrwolf]

I'm landing this without a formal review since this provider is still unused/unreleased and in the early dev/exploration phase.