common: Add new StepDownload to DRY up downloads
builder/virtualbox: switch download guest additions to use new common
builder/virtualbox: remove unused function
builder/vmware: switch to common download step
builder/virtualbox: iso_urls for multiple ISO urls [GH-110]
builder/vmware: support iso_urls for multiple URLs [GH-110]
post-processor/vagrant: use common.PackerConfig
post-processor/vagrant: process user variabels [GH-295]
packer: shuffle some tests around to where they make more sense