Don't create containers on "halt" command (#102)

When running `lxdock halt` after having only a subset if our containers
being created, we would end up in this annoying situation where missing
containers would be created and then halted. Not anymore.

lxdock/container.py

@@ -70,6 +70,10 @@ def destroy(self):
 
     def halt(self):
         """ Stops the container. """
+        if not self.exists:
+            logger.info("The container doesn't exist.")
+            return
+
         if self.is_stopped:
             logger.info('The container is already stopped.')
             return

tests/integration/test_container.py

@@ -65,6 +65,13 @@ def test_can_destroy_a_container_and_run_this_action_for_a_container_that_does_n
         container.destroy()
         assert not container.exists
 
+    def test_halting_a_container_doesnt_create_it(self):
+        container_options = {
+            'name': self.containername('doesnotexist'), 'image': 'alpine/3.6', }
+        container = Container('myproject', THIS_DIR, self.client, **container_options)
+        container.halt()
+        assert not container.exists
+
     def test_can_halt_a_container_that_is_running(self, persistent_container):
         persistent_container.halt()
         assert persistent_container._container.status_code == constants.CONTAINER_STOPPED