migration_manager: Use in-place value factory
migration_manager: Use in-place value factory

The factory is purely a state-less thing, there is no difference what
instance of it to use, so we may omit referencing the storage_service
in passive_announce

This is 2nd simple migration_manager -> storage_service link to cut
(more to come later).

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla