core: Force removing of zombie tasks

The following patch removes zombie tasks from db,
In the following way:
A. It tries to stop the task - if fails to remove, it changes the task state to ended,
and its status to finish - this will present a chance to clear the task
b. For finished tasks - tries to clean the task at vdsm  - and removes it from DB.

Both a and b are achieved by introrducing new stopTask and ClearTask method that
accept boolean flags for "force" behavior

Bug-Url: https://bugzilla.redhat.com/907465
Change-Id: I8935ae5bced057aa3f37bf7ef09d3f2cf153571e
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>