qa/tasks: introduce Thrasher base class

qa/tasks: introduce Thrasher base class

* Introduced a Thrasher base class.
* Updated thrashers to inherit from Thrasher.
* Replaced the magic variable e with Thrasher.exception as per the discussion.
  Now the exception variable sets by default as the thrashers are inheriting
  from the Thrasher class.

Fixes: https://github.com/ceph/ceph/pull/28378#discussion_r309337928
Fixes: https://tracker.ceph.com/issues/41133
Signed-off-by: Jos Collin <jcollin@redhat.com>