Fixing for-each

 * Fixing for-each according to comments in https://review.openstack.org/#/c/140327/
 * Normalizing for-each output
 * Better index checking
 * Move for_each module to mistral/workflows/
 * Unit tests on for_each module

Partially implements mistral-dataflow-collections

Change-Id: I5c023573f0950cb599a99d0bd48dba95f3c8dc40
Merge "Fixing for-each"