Task: Enanched task utility to run completed callback function.
Task: Enanched task utility to run completed callback function.

This patch enhances task utility by adding completed call back
function, this callback function is executed immediately after
task.Done is invoked.

Change-Id: I1a52d006aecb3bfbc9228890f72ed417bf17a509
Signed-off-by: Darshan N <d2c6d450ab98b078f2f1942c995e6d92dd504bc8@gmail.com>

skyrings/skyring-common,skyrings/skyring-common