Recursive generation of YAQL objects

So, i've run into this same issue myself.

From my investigation i believe it's caused by mistral not updating the context used for expression evaluation after each publish statement is evaluated. Instead the context is created at the beginning of the publish block and then merged into the global context upon completion of the task.

The task begins publishing variables here:
https://github.com/openstack/mistral/blob/master/mistral/engine/tasks.py#L164

Then a context is created and evaluation begins:
https://github.com/openstack/mistral/blob/master/mistral/workflow/data_flow.py#L187-L217

Evaluation is triggered by this line:
https://github.com/openstack/mistral/blob/master/mistral/workflow/data_flow.py#L209

Evaluation is done by the following:
https://github.com/openstack/mistral/blob/master/mistral/expressions/__init__.py#L88-L103

It's in this final code snippet where you can see that the evaluation simply creates a new data structure (dict) and does not merge the result back into the current context being used for evaluation.

This is my first time looking at the code base so i may be off in left field.

Any feedback is appreciated.

I have already done this in another project with variables generated from Jinja2. Basically, what we could do is:
- extract all of the variables inside the YAQL code
- generate a dependency graph using the variable names, it is possible to detect dependency loops at the same time. Raise an error if there's a loop (it could be possible to support loops, but I don't think it's good idea).
- compute the variables in the order of the dependency graph, recursively

I just need to spend more time studying the code that interprets the "publish" sections in the DSL.

nmaludy:

https://github.com/openstack/mistral/blob/master/mistral/expressions/__init__.py#L88-L103

'evaluate_recursively' does not recursively evaluates the YAQL code as we would like. It just goes through the data dictionary recursively.

It allows you to put YAQL in non-string types:

```yaml
version: 2.0

my_workflow:
  tasks:
    start:
      action: std.noop
      publish:
        my_first_variable:
          child_variable: <% $.stuff + 1 %>
```
instead of only being able to use YAML strings in published variables like this:
```yaml
version: 2.0

my_workflow:
  tasks:
    start:
      action: std.noop
      publish:
        my_first_variable: <% dict(child_variable=>$.stuff + 1) %>
```