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>
Merge pull request #249 from ceph/wip-9557

tasks: add mds_auto_repair

Reviewed-by: John Spray <john.spray@redhat.com>