Multi-Node usage for Convoy Extension

[julianfegert]

When you create a product in Paymenter, you can only select one host in the Convoy extension for paymenter. If this host is fully booked, an error appears: exceeds the nodes limit.

The Convoy extension for paymenter should use the next free host if several hosts are stored in Convoy.

For example, 2 hosts are stored in Convoy, e.g. pve06 and pve08. The convoy extension for paymenter is set to pve06. pve06 is fully utilized, then the extension should not issue an error but create the vm on pve08. an error message should only be issued if there is no free capacity on any existing node.