fix #1358: ui: show clustername besides 'Datacenter' node, if available

fix #1358: ui: show clustername besides 'Datacenter' node, if available

If we get the cluster name (successful login with '/' Sys.Audit
permissions) then display it in the resource tree's root node.

This updated on login and all ticket refreshs (every 15 minutes).
I currently have no functionallity to refresh it actively on cluster
create over WebUI, as it's not a straight forward change there.
Further, this is something which does not changes often (in
production), and we cannot detect CLI or API triggered (from non-pve
clients) cluster creations anyway.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>