core:[ExternalTasks] Cannot add sub-step under...

[ExternalTasks] Cannot add sub-step under existing (parent) step

1) Fixing mandatory parameters step.parentStep.id => parentStep.id
2) Adding handling to the add method to inject the step parent

Change-Id: Ibc610d6f295a688ceec1b038107c07d43390b431
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=988082
Signed-off-by: Eli Mesika <emesika@redhat.com>