Use linked base images for parallels

Difference between linked and full clones:

* Linked clone creation is extremely faster than the full cloning, because there is no image copying process.
* Linked clone requires much less disk space, because initially its hard disk image is less than 1Mb (it is bound to the parent's snapshot).
* Full clone is a full image copy, which is independent from the box. Linked clones are bound to the specific snapshot of the box image. It means that box deletion will cause all its linked clones being corrupted. Then please, delete your boxes carefully!

Vagrantfile

@@ -141,6 +141,10 @@ Vagrant.configure(VAGRANTFILE_API_VERSION) do |config|
         v.cpus   = configuration['VM']['cpu']
         v.update_guest_tools = true
 
+        # Used linked base images, save disk space (Parallels >= 11)
+        # see https://parallels.github.io/vagrant-parallels/docs/configuration.html
+        v.linked_clone = true
+
         if configuration['VM']['gui']
             v.customize ["set", :id, "--startup-view", "window"]
             v.customize ["set", :id, "--on-window-close", "suspend"]