createlinks, actions: backup secrets.tdb in config backup. Refs #2653
Store SID in Config DB. Refs #2626

On *-update and migration events, store Samba SID in smb/Sid prop. As
soon as hostname changes, implant the stored SID in secrets.tdb, to
avoid generating a new one.
Drop all sambaDomain objects in LDAP before starting migration. Refs #2626

All domains will be recreated with the migrated SID.