Added support for com2sec source definitions for the RO community.

The value defaults to an array containing 'default', but can be set to hostnames, IP addresses, or IP ranges as defined in the net-snmp docs: http://www.net-snmp.org/docs/man/snmpd.conf.html#lbAL

The default value should prevent any issues with upgrading puppet-snmp, as the end-result configuration should be the same in that case.

I understand this may somewhat conflict with the eventual intended functionality of ro_network, but that's why I used a different variable.