CBQE-0: cleanup code in xdcrbasetests.py.

1. shutdown taskmanager before collecting logs/data files.
2. default get-collect-logs = False.
3. Refactor code.

Change-Id: I2ac5b94a955690ff8562e973adab8884ede84afa
Reviewed-on: http://review.couchbase.org/41076
Reviewed-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>
Tested-by: Sangharsh Agarwal <sangharsh.agarwal@globallogic.com>