Add `vagrant smartos latest` to show name of latest platform image

[refs #4]
Sort platform images
Allow for 'latest' as platform_image

[refs #4]
Inline unnecessary downloader methods
zones command needs ui for writing failed subcommands

-- needs refactor
in-place refactor
zone user/group creation should be owned by relevant classes

Responsibilities should be delegated to relevant objects.
Create zone vagrant group with a ZoneGroup. Create zone vagrant user
with a ZoneUser.