SSM Notification config fix

[vsimakhin]

fix the validator, otherwise it raises the error isinstance() arg 2 must be a type or tuple of types

File ".../templates/common/ssm/common_ssm_v1.py", line 165, in main
    NotificationType=utils.parse_arg(notification_config, name="type")
  File ".../lib/python3.6/site-packages/troposphere/__init__.py", line 341, in __init__
    super(AWSProperty, self).__init__(title, **kwargs)
  File ".../lib/python3.6/site-packages/troposphere/__init__.py", line 124, in __init__
    self.__setattr__(k, v)
  File ".../lib/python3.6/site-packages/troposphere/__init__.py", line 189, in __setattr__
    if not isinstance(v, tuple(expected_type)) \
TypeError: isinstance() arg 2 must be a type or tuple of types

[markpeek]

Thanks!