Update install instructions

❯ python3 -m openappstack varac-oas create --create-droplet --hostname varac-test --ssh-key-id 407 --create-domain-records varac-test.openappstack.net

Creating droplet
Created droplet id: 3276
Waiting for droplet 3276 to reach running state...
Waiting for ssh to become available on ip 213.108.108.204
SSH became available on ip 213.108.108.204
Creating domain record
Traceback (most recent call last):
  File "/usr/lib/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/varac/projects/work/greenhost/openappstack/openappstack/__main__.py", line 360, in <module>
    main()
  File "/home/varac/projects/work/greenhost/openappstack/openappstack/__main__.py", line 185, in main
    args.func(clus, args)
  File "/home/varac/projects/work/greenhost/openappstack/openappstack/__main__.py", line 246, in create
    args.domain, clus.ip_address, subdomain=args.subdomain)
  File "/home/varac/projects/work/greenhost/openappstack/openappstack/__main__.py", line 320, in create_domain_records
    data=droplet_ip, record_type='A', update=True)
  File "/home/varac/projects/work/greenhost/openappstack/openappstack/cosmos.py", line 78, in create_domain_record
    record_type=record_type)
  File "/home/varac/projects/work/greenhost/openappstack/openappstack/cosmos.py", line 172, in get_domain_record_by_name
    records = get_domain_records(domain=domain)
  File "/home/varac/projects/work/greenhost/openappstack/openappstack/cosmos.py", line 160, in get_domain_records
    response = request_api('domains/{0}/records'.format(domain))
  File "/home/varac/projects/work/greenhost/openappstack/openappstack/cosmos.py", line 59, in request_api
    response.status_code, response.text)
requests.exceptions.HTTPError: [Errno WARNING: Got response code ] 500: '{"error":"Domain not found"}'

If I got it wrong (as OAS dev!) we should better document the usage of DOMAIN_NAME and --subdomain, and give an example in the help output.