Fix passing workflow input via UI
Fix passing workflow input via UI

 * When we are using UI to pass workflow input
   to execution, we always pass it as json string.
   We need to prevent using extra json.dumps on string.

Change-Id: Ie11c342b305a1359f9a28d354230f515d36e7a0c

StackStorm/python-mistralclient,StackStorm/python-mistralclient,openstack/python-mistralclient,openstack/python-mistralclient