Update power_on method to return self instead of task object

Change-Id: I9b382c0ea859ffe9cfe1bb86bab3ef407b330737
Update power_off method to return self instead of task object

Change-Id: Ib533974441359cba0746bf74a00cd0764348f3a7
Update add_network_by_name method in VApp OM to return VApp object instead of task

Change-Id: Ibfc441a02381c26c145f60277ee211ea610b1971