OCS CLI tools crash on empty hosts config

[cnweaver]

If the site YAML contains the hosts key but its value is empty (implicitly null), ocsbow, ocs-local-support, etc. crash:

$ cat ${OCS_CONFIG_DIR}/default.yaml
# Site configuration for a fake observatory.
hub:

  wamp_server: ws://localhost:8001/ws
  wamp_http: http://localhost:8001/call
  wamp_realm: test_realm
  address_root: observatory

hosts:

$ ocsbow status                                                          
Traceback (most recent call last):
  File "$VIRTUAL_ENV/bin/ocsbow", line 8, in <module>
    sys.exit(main())
  File "$VIRTUAL_ENV/lib/python3.7/site-packages/ocs/ocsbow.py", line 821, in main
    args, site_config = get_args_and_site_config(args)
  File "$VIRTUAL_ENV/lib/python3.7/site-packages/ocs/ocsbow.py", line 100, in get_args_and_site_config
    args_ = ocs.site_config.parse_args(agent_class=agent_class, parser=parser, args=args)
  File "$VIRTUAL_ENV/lib/python3.7/site-packages/ocs/site_config.py", line 699, in parse_args
    site, host, instance = get_config(pre_args, agent_class=agent_class)
  File "$VIRTUAL_ENV/lib/python3.7/site-packages/ocs/site_config.py", line 422, in get_config
    site_config = SiteConfig.from_yaml(site_file)
  File "$VIRTUAL_ENV/lib/python3.7/site-packages/ocs/site_config.py", line 61, in from_yaml
    self = cls.from_dict(data)
  File "$VIRTUAL_ENV/lib/python3.7/site-packages/ocs/site_config.py", line 50, in from_dict
    for k, v in data.get('hosts', {}).items():
AttributeError: 'NoneType' object has no attribute 'items'

Such a config file could be viewed as ill-formed and rejected, but it would be nice if this were diagnosed with a clear message to the user instead of crashing with an internal error, as using YAML makes this mistake very easy to make.