suites/fs: add a standbyreplay configuration

*** This PR depends on teuthology PR https://github.com/ceph/teuthology/pull/296 merging first ***

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>