tests: Thrasher: eliminate a race between kill_osd and __init__

tests: Thrasher: eliminate a race between kill_osd and __init__

If Thrasher.__init__() spawns the do_thrash thread before initializing the
ceph_objectstore_tool property, do_thrash races with the rest
of Thrasher.__init__() and in some cases do_thrash can call kill_osd() before
Trasher.__init__() progresses much further. This can lead to an exception
("AttributeError: Thrasher instance has no attribute 'ceph_objectstore_tool'")
being thrown in kill_osd().

This commit eliminates the race by making sure the ceph_objectstore_tool
attribute is initialized before the do_thrash thread is spawned.

Fixes: http://tracker.ceph.com/issues/18799
Signed-off-by: Nathan Cutler <ncutler@suse.com>