core: RFE-Adding External Tasks Support..

Enable plug-in to inject tasks to the engine application using the REST
API, change their statuses and track them from the UI.
A task may have other nesting sub-tasks under it.

more info :
http://www.ovirt.org/Features/ExternalTasks
http://www.ovirt.org/Features/Design/DetailedExternalTasks

This patch adds jobId and stepId to VdcActionParametersBase

Change-Id: I66797a54605eac45eabd17e26c7826484725cc23
Signed-off-by: Eli Mesika <emesika@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=872719