suites/fs: add a standbyreplay configuration

A place for any tests which should be run with
standbyreplay MDSs in play.  Initially just
the mds_journal_migration task, to cover
the #8811 case.

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #62 from ceph/wip-standbyreplay

suites/fs: add a standbyreplay configuration