Calling stacker without arguments will crash instead of printing help

With stacker 1.7.0 and Python 3.7.3:

```
Traceback (most recent call last):
File "./venv/bin/stacker", line 8, in
args = stacker.parse_args()
File "/home/danielkza/cobli/deploy/cloudformation/venv/lib/python3.7/site-packages/stacker/commands/stacker/base.py", line 115, in parse_args
args.environment.update(args.cli_envs)
AttributeError: 'Namespace' object has no attribute 'environment'
```