Improved devstack VirtualBox script

* Allow `start` and `headless` params to be provided to the build task,
  causing the VM to be started upon creation, and if so, whether to do
  so in headless mode
* Add a `destroy` task that stops a VM and deletes it altogether
  (drives included!)
* Add `start_vm` and `stop_vm` task helpers
* Add a few helpers, like `vm_off` which returns whether a VM is fully
  powered off, and `vm_state` which returns the VMState string for the
  provided VM
Globalize args to apt-get commands
Major overhaul of apt.add_repo; add add_key_url

* apt.add_key_url downloads a apt key from the specified URL and uses
  apt-key to add it
* apt.add_repo:
  - now supports deb-src sources
  - now supports specifying a name for the purposes of writing to a
    file under /etc/apt/sources.list.d/
  - now supports removal of a source from any of sources.list and
    the /etc/apt/sources.list.d/*.list
* apt.remove_repo added as an alias to add_repo(remove=True)
  (avoiding DRY due to overlapping logic)