qa/workunits/restart: Add test to check backtrace

qa/workunits/restart:  Add test to check backtrace

This script uses the python bindings to libcephfs and rados
to create files and check the correctness of the backtrace
written to the 'parent' xattr on the first object (if its
a file) or inode (if its a dir).  The script includes test cases
that kill the mds at specific kill points and restart it through
teuthology using the teuthology restart task.

Signed-off-by: Sam Lang <sam.lang@inktank.com>