Inquiries can be created with an empty schema

Right now, Inquiries can be defined with an empty schema:

version: '2.0'

examples.mistral-ask-basic:
    description: A basic Mistral workflow illustrating the use of Inquiries
    type: direct
    output:
        result: <% task(task1).result.response %>
    tasks:
        task1:
            action: core.ask
            input:
              route: developers
              schema: {}

Since there are no properties to iterate over, at a minimum this causes issues at respond time:

(virtualenv) vagrant@st2dev:~/st2$ st2 inquiry get 5a05626b32ed3550e323c7a3
+----------+--------------------------+
| Property | Value                    |
+----------+--------------------------+
| id       | 5a05626b32ed3550e323c7a3 |
| roles    |                          |
| users    |                          |
| route    | developers               |
| ttl      | 1440                     |
| schema   |                          |
+----------+--------------------------+
(virtualenv) vagrant@st2dev:~/st2$ st2 inquiry respond 5a05626b32ed3550e323c7a3
ERROR: 'NoneType' object is not iterable

I can get the inquiry to pass using the -r option but the client still breaks:

(virtualenv) vagrant@st2dev:~/st2$ st2 inquiry respond 5a05626b32ed3550e323c7a3 -r {}
ERROR: 'properties'

We should add some logic into the inquirer runner to perform some sanity checks on a user-provided schema to ensure some things:

• Isn't empty
• Defines an object with properties
• Those properties don't define further objects (no nesting)