Set PI_NODE during redundancy setup

[fredreichbier]

privacyIDEA supports the PI_NODE/PI_AUDIT_SERVERNAME config option to assign a name to the local privacyIDEA node. This might be handy in a redundant setup. Our redundancy setup should ask for node names and set them accordingly.

[fredreichbier]

We should then also set PI_NODES, to integrate nicely with periodic tasks.

[cornelinux]

For simplicity we could start with

PI_NODE = "node1"
PI_NODES = ["node1", "node2"]
PI_AUDIT_SERVERNAME = "node1"

and

PI_NODE = "node2"
PI_NODES = ["node1", "node2"]
PI_AUDIT_SERVERNAME = "node2"

(no user interaction)