vshield task manager: abort tasks in stop() on termination

This patch kills the manager thread, and aborts active tasks
rather than sending an exception to the manager thread and
have it do the abort on termination.

Unit tests involving vshield task manager might take longer
as a side effect of this patch.

Related-bug: #1282452

Conflicts:

        neutron/tests/unit/nicira/test_vcns_driver.py

Change-Id: I9e9e41ce7e8969a2ea51bfce96b1303125a24308
(cherry picked from commit 433287b4f51e45fbb7a73f68dd63b433ea5d9066)
Merge "vshield task manager: abort tasks in stop() on termination" into stable/havana