Runs `helm version` in helmexec.New, and exposes a method on Interface to allow other packages to use the detected version. Preserves compatibility with previous HELMFILE_HELM3 mechanism.
Resolves#923
* fix: Fix `needs` to work for upgrades and when selectors are provided
Fixes#919
* Add test framework for `helmfile apply`
* Various enhancements and fixes to the DAG support
- Make the order of upgrades/deletes more deterministic for testability
- Fix the test framework so that we can validate log outputs and errors
- Add more test cases for `helmfile apply`, along with bug fixes.
- Make sure it fails with an intuitive error when you have non-existent releases referenced from witin "needs"
Andrew Drake
KUOKA Yusuke