builder/virtualbox: Add an `export_opts` option

Allows arbitrary arguments to VBoxManage export, such as product info (--product, --vendor, etc).
Pass export_options individually to VBoxManage export
Move to an array of strings for a deterministic option separation
Log export command being executed
Properly apply templates to export opts
Fix export opts tests
Formatting
Merge pull request #945 from benlangfeld:feature/vb_output_opts (manually)