Virtualbox Scripts minor enhancements

1.split prepare-environment.sh into 3 actions. 2. add VBoxManage path to
PATH. 3. create_vm() function able to create VMs with different OS types

Implements blueprint virtualbox-scripts-enhancements

Change-Id: I4d34ba3f39af8becbbe8dc700d57dd5ea8cce0ee
Merge "Virtualbox Scripts minor enhancements"