fix(destroy,halt): remove tasks status polling

Since the tasks API endpoint does not exist anymore (kaorimatz/fog-scaleway#7),
polling for the task will result in an error even if the actual task succeeded.

While tasks-related code should be removed from fog-scaleway,
setting async to true for the server.terminate and server.poweroff methods
will bypass this polling.

Fixes kaorimatz#9.

lib/vagrant-scaleway/action/destroy_server.rb

@@ -17,7 +17,7 @@ def call(env)
             server.destroy
           rescue Fog::Scaleway::Compute::InvalidRequestError => e
             if e.message =~ /server should be stopped/
-              server.terminate(false)
+              server.terminate(true)
             else
               raise
             end

lib/vagrant-scaleway/action/stop_server.rb

@@ -14,7 +14,7 @@ def call(env)
             env[:ui].info(I18n.t('vagrant_scaleway.already_status', status: env[:machine].state.id))
           else
             env[:ui].info(I18n.t('vagrant_scaleway.stopping'))
-            server.poweroff(false)
+            server.poweroff(true)
           end
 
           @app.call(env)