Update Vagrant integration

- most of the logic behind main Vagrantfile has been moved to
  inventory-based Vagrantfile, which now can be used as standalone (while
  being in the main project directory). Specify "VAGRANT_VAGRANTFILE"
  variable to use it directly, otherwise main Vagrantfile will select the
  default one;

- Vagrant now should correctly update 'inventory-vagrant' symlink if
  different inventory is selected. Do not change inventories while Vagrant
  environment is active;