Server Pools: Backup servers

[samuelwei]

Is your feature request related to a problem? Please describe. In case all servers are unavailable, e.g. due to a DC location issue, no meetings linked to this server pool can be started.

Describe the solution you'd like Keep the current server pool server list. Add an additional list of backup server. Only if no server of the main list is available, use the server with the lowest usage of the backup server list.