Fix the DeleteAllPackageVersionsTask to 1) call DeletePackageVersionTask.ValidateArguments() which does some initialization that was missing 2) require a Reason for the deletion
Also pass through the ConfigFile and EnvironmentName arguments which DeletePackageVersionTask requires and the parent task magically gets due to the galops powershell alias.
Going with .Execute() for fuller consistency with how commands are normally invoked and easier debugging.
Apply Andrew's Code Review Feedback for DeleteAllPackagesVersionsTask. Untested, but should hopefully work next time someone needs to delete a package. If not, you can revert this change? :)
Merge pull request #1642 from NuGet/tilovell/FixBrokenDeleteFullPackageTask

Tilovell/fix broken delete full package task