Mistral wf stuck in RUNNING if the sum of task's output is more than limit

[Oleg Ovcharuk]

If each task's published output is fit in execution_field_size_limit_kb, but the sum of them is not, mistral's workflow could stuck in RUNNING state.

Look at this example:

execution_field_size_limit_kb = 500

wf_field_limit:
  tasks:
    task1:
      action: std.http
      input:
        url: <% "https://en.wikipedia.org/wiki/QWERTY" %>
      publish:
        out1: <% task().result %>
    task2:
      action: std.http
      input:
        url: <% "https://en.wikipedia.org/wiki/QWERTY" %>
      publish:
        out2: <% task().result %>

size(out1) == size(out2) == ~ 400 KB
but wf's output, which includes both out1 and out2 is bigger than 500KB limit.

expected result: there is error message in logs and workflow's state is ERROR
actual result: there is error message in logs, but workflow's state stuck in RUNNING

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/608469