virtualbox: use local VBoxGuestAdditions.iso by default

fixes #1123
updated docs
common: add test for empty val
Merge pull request #1132 from rasa/1123-use-local-vboxguestadditions-iso

virtualbox: use local VBoxGuestAdditions.iso by default
Update CHANGELOG
Force LANG=C before appending rest of environment, as only first occurence is used
Merge pull request #1145 from jsoriano/fix-env-order-lang-c

Force LANG=C before appending rest of environment
provisioner/file: set file/directory permissions

fixes #1064