Revert "Remove Virtualbox provider."

This reverts commit 5cadc401c78377c1b744d9a6bd0ff0a12b61da2f.
Do not modify /etc/resolv.conf on first boot

* /etc/resolv.conf is cleaned up in the end of stemcell building
* agent will restart networking after writing /etc/resolv.conf

Signed-off-by: dmitriy kalinin <dmitriy@pivotallabs.com>