Allow setting MIBDIRS per generator module

[SuperQ]

A common source of problems generating snmp.yml configurations is that some MIBs produce conflicts. For example, SNMPv2-TC and RFC1213-MIB both define DisplayString. There are many such examples of these kinds of conflicts.

In order to work around this, we can allow each generator module config to include a mibdirs specification that will override the default MIBDIRS environment variable. This way each module can have an isolated instance of the net-snmp parsing.