mds_thrash: PEP8-ize whitespace

...so that I can edit the code in a python IDE without
it lighting up like a christmas tree!

Signed-off-by: John Spray <john.spray@inktank.com>
mds_thrash: Refactor gevent usage + get traceback

This simplifies the code to make MdsTrash be a greenlet
(as it logically is) rather than encapsulating one that
gets started in __init__ (spawning threads in constructors
is evil).

With this done, do_thrash is called from _run inside an
exception handler that will give us full tracebacks
if something bad happens.

Signed-off-by: John Spray <john.spray@inktank.com>
mds_thrash: Fix a potential getitem on None

get_mds_status returns None for things it can't see,
so have to check for Noneness on all its outputs.

Signed-off-by: John Spray <john.spray@inktank.com>
Merge pull request #211 from ceph/wip-7554

mds_thrash #7554