tasks: add mds_auto_repair

New CephFS tests for MDS's auto repair functions. (So far the only
test case is verify/repair backtrace on fetch dirfrag)

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Doing 500 separate run_shell calls is slow if the test runner is remote (like my workstation) from the lab nodes, because of the SSH setup/teardown overhead.  Elsewhere in places like open_n_background I use some python snippets to do many remote operations in a single SSH command because it's faster.