Allow to set more snmpv3 credentials

[stevie-sy]

The situation is now following:

• for v2 traps it is possible to set multiple community strings for traps we accept.
• for v3 traps you can configure only one v3 user wich credentials which everybody of our colleagues has to use for their maintaining hardware (storage, firewall, switches etc.). So it would be great to get the possibility to set more v3 credentials in one plugin definition.

The only workarround would be to configure more input plugins with diffrent v3 users incl. credentials. And every plugin definition has to listen to a diffrent port. If not we get an error message that this is not possible. But this isn't really nice if we can not use the standard trap port.

Why this would be great?

1. For security reason! With the situation now, every department (storage, server, network, firewall) has to use the same credentials and this should be a no go! And we have some devices where the snmp credentials are the same for sending traps and requesting infos.
2. Also a lot of our colleagues want to use - of course for security reason - the highest possible security protocol. e.g. AES256 / SHA256. But not every hardware manufacturer support the same protocols. So If we can only configure one v3 user (for everybody) you have to choose the absolute minium protocol (which may be insecure). That's often sha1/aes. But some devices like firewall doesn't support this old protocols any more.