Fix input on execution create

This fix apparently broke it
https://review.openstack.org/#/c/121510/4/mistral/api/controllers/v2/execution.py

Added the dict to unit-tests.
Tested with client: mistral execution-create wb1.wf1 '{ "str1":"foo", "str2":"bar" }'

Change-Id: I75be2ab35210f1a7f1bdca1a18c1f7f375862d27