Refactor aux port allocation

This move the allocation of aux port to the base vm.
Also now the free of console port during the close is in the
base VM.

An aux port is allocated to the docker container but not used
for the moment.

Ref https://github.com/GNS3/gns3-gui/issues/1039

tests/modules/docker/test_docker_vm.py

@@ -54,6 +54,7 @@ def test_json(vm, project):
         'vm_id': vm.id,
         'adapters': 1,
         'console': vm.console,
+        'aux': vm.aux,
         'start_command': vm.start_command,
         'environment': vm.environment,
         'vm_directory': vm.working_dir