Add NetBSD guest support
synced_folders/rsync: output directory under watch for rsync-auto
Install Bundler 1.5.2 for tests
providers/virtualbox: validate missing keys [GH-2831]
commands/init: update template to use strings, not symbols
provisoiners/docker: only add user to group if not in it [GH-2838]
Merge pull request #2819 from stoned/guest-netbsd

Add NetBSD guest support
Update CHANGELOG
guests/tinycore: TinyCore Linux support