Add flag to disable upgrade tests for renamed or removed values files

[jlegrone]

Right now switching on the --upgrade flag can lead to cases where it's not possible to get ct to succeed, based on the contents of a previous revision.

The upgrade flag uses the previous revision's ci values files instead of the current revision's values files, and thus if the previous revision has a ci values file that does not support upgrades (example), it becomes impossible to get ct to pass unless you bump the version in Chart.yaml a major version.

One approach to this problem is to add a flag that will tell ct not to pay attention to values files from the previous revision that have been removed or renamed in the current revision. This will allow simply deleting bad values files at the current revision to get builds passing.

A downside to this would be that some legitimate failure modes could be missed during renames or that folks might be encouraged to simply delete test files when there could be a good reason they're failing.