restapi: 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

Change-Id: Id1b95a094dc586e6ebbdacd44e0a034e91601952
Signed-off-by: Eli Mesika <emesika@redhat.com>
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=872719