Adds missing "params" attribute for creating a Mistral workflow execution

The v2 API of Mistral service have "params" attribute
for creating workflow execution [1]. This attribute
is a json object for workflow type-specific parameters.
For example, the "env" (environment) dict can be used
for installation specific inputs (dev/staging/etc..).
We are using such env's to split environments
but having the same logic in workflows definitions.

[1] https://docs.openstack.org/mistral/latest/api/v2.html#executions

Change-Id: Ic317b10c7b0053a8103ea2060491e437efc6898f
Story: 2005232
Task: 30014

openstack/workflow/v2/execution.py

@@ -43,6 +43,8 @@ class Execution(resource.Resource):
     #: A JSON structure containing workflow input values
     # TODO(briancurtin): type=dict
     input = resource.Body("input")
+    #: An optional JSON structure containing workflow type specific parameters
+    params = resource.Body("params")
     #: The output of the workflow
     output = resource.Body("output")
     #: The time at which the Execution was created