get_action_list: improve generated JSON output
get_action_list: improve generated JSON output

* use ',' instead of ', ' as separator, this way the trailing
  whitespaces at the end of a line will be removed

* use an indent of 8 instead of 4, this way it is possible to paste
  the generated JSON output into the mapping.json file

Change-Id: I5232490afabf5024e573109d3fc8ba3a6a003a47

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