Cleaned up snmpd::packetfilter

- Switched to using firewall resources directly
- Fixed a rule duplication issue

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>
Replaced snmpd::configuration define with snmpd::config class

There was no benefit in having snmpd.conf managed by a defined resource, so
switched to using a class instead.

Signed-off-by: Samuli Seppänen <samuli@openvpn.net>