Ref T2015. Ref T1049. This allows users to select different shells when executing commands via Harbormaster. Primarily this is to address an issue where running native commands under Powershell on Windows will swallow up the exit code, causing false positives when native commands fail. In order for native command exit codes to be reported correctly, they need to be run under "Windows Command Prompt".

Tested this behaviour locally with a build plan that runs commands under various shells.