Data source lookups should return a default value when the source in unavailable

[blakeneyops]

It is common in CI environments that we may be testing stacks that depend on each other, which have not yet been applied. Currently, this causes some not very gracefully handled errors and requires some overriding of build data.

I think two features would be useful here:

• Firstly, it would be good to have a flag that disables all external lookups and simply returns something like "covalence_generated". This would be useful for scenarios where we simply want to test that a tool can execute the provided code and the actual data is irrelevant.
• Second, it would be good if Covalence additionally returned a canned response in the event of executing a plan where the data source either doesn't respond or isn't present.

Additionally, there should be better error handling when a lookup is required and does not resolve.

These modes could be feature flagged with an environment variable to provide some flexibility in how Covalence responds under specific conditions.