Fix the 'params' field of the workflow execution REST resource
Fix the 'params' field of the workflow execution REST resource

* API reference is also build based on this docstring

Change-Id: I2efb3cffc93c926f6f980f6803e4f781322ae1fc
Closes-Bug: #1690711

StackStorm/mistral,StackStorm/mistral,openstack/mistral,openstack/mistral