Fix workflow update API
Fix workflow update API

 * workflow update should not update spec if we
   didn't pass the definition property. Otherwise
   we get YAML parser error.

Change-Id: Ib89798427649c28d7da81a0a24d0aaf973a31148

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