Fixed build-snmp-config to work with non-cisco

Added the CASA mibs.  Made it default to using Cisco when it can't find
the router type (shouldn't hurt, cpu graphs/usage just won't work)

I may add support for juniper and other vendors later.  I thought about
adding support for net-snmp, but their oids don't track CPU in a good
way for our use.  It doesn't seem like anyone exposes as many cpu
counters as cisco, so most people will probably just hack the same oid
into both 1min and 5min.